THE  EVOLUTION  OF  MAN. 


HAECKEL'S  EVOLUTION  OF  MAN.  PLATE  I. 

DEVELOPMENT  OF  THE  FACE  (THIRD  STAGE). 

EXPLANATION    UK    CHAP.    XXI. 


M.    Man.  B.   Bat.  S.    Sheep.  C.  Cat. 


THE 

EVOLUTION    OF   MAN 

A  POPULAR  EXPOSITION 

OF    THE 

PRINCIPAL  POINTS  OF  HUMAN  ONTOGENY  AND  PHYLOGENY. 

FROM    THE    GERMAN    OF 

ERNST    HAECKEL, 

PROFESSOR   IN   THE   FNIVEBSITY   OF  JENA, 
AUTHOR     OF     "THE     HISTORY     OF    C  E  E  A  T I  O  N."    BTC. 


IN    TWO    VOLUMES. 
VOL.  I. 


NEW  YORK: 
D.    APPLETON    AND    COMPANY, 

72    FIFTH    AVENUE. 
1898. 


Authorized  Edition. 


SAOTA  BARBARA  COLLEGE  LIBRARY 

v  / 


CONTENTS  OF  VOL.  I. 


List  of  Plates  ...             ...  ...             ...             ...             ...          xiv 

List  of  Woodcuts    ...             ...  ...             ...             ...                    xv 

List  of  Genetic  Tables  ...  ...             ...             ...             ...         xviii 

Preface  to  the  First  Edition  ...             ...             ...                    xix 

Preface  to  the  Third  Edition  ...             ...             ...             ...        xxvii 

Prometheus             ...             ...  ...             ...             ...              xxxvii 

Faust                               ...  ...             ...             ...             ...     xxxvii 


CHAPTER  I. 

THE  FUNDAMENTAL  LAW  OF  THE  EVOLUTION  OF  ORGANISMS. 

General  Significance  of  the  History  of  the  Evolution  of  Man. — Ignor- 
ance of  it  among  the  so-called  Educated  Classes. — The  Two 
Branches  of  the  History  of  Evolution. — Ontogeny,  or  the  History 
of  Germs  (Embryos),  and  Phylogeny,  or  the  History  of  Descent  (or 
of  the  Tribes). — Causal  Connection  between  the  Two  Series  of 
Evolution. — The  Evolution  of  the  Tribe  determines  the  Evolution 
of  the  Germ. — Ontogeny  as  an  Epitome  or  Recapitulation  of  Phy- 
logeny. The  Incompleteness  of  this  Epitome. — The  Fundamental 
Law  of  Bicgeny. — Heredity  and  Adaptation  are  the  two  Formativu 
Functions,  or  the  two  Mechanical  Causes,  of  Evolution. — Absence 
of  Purposive  Causes. — Validity  of  Mechanical  Causes  only. — Sub- 
stitution of  the  Monistic  or  Unitary  for  the  Dualistic  or  Binary 
Cosmology. — Radical  Importance  of  the  Facts  of  Embryology  to 
Monistic  Philosophy. — Palingenesis,  or  Derived  History,  and  Keno- 
Kcuesis,  or  Vitiated  History. — History  of  the  Evolution  of  Forms 
and  Functions. — Necessary  Connection  between  Physiogeny  and 


VI  CONTENTS. 


Morphogeny.— The  History  of  Evolution  as  yet  almost  entirely 
the  Product  of  Morphology,  and  not  of  Physiology. — The  History 
of  the  Evolution  of  the  Central  Nervous  System  (Brain  and  Spinal 
Marrow)  is  involved  in  that  of  the  Psychic  Activities,  or  the  Miud  1 


CHAPTER  II. 

THE  EARLIER  HISTORY  OF  ONTOGENY. 
CASPAR  FEIEDEICH  WOLFF. 

The  Evolution  of  Animals  as  known  to  Aristotle. — His  Knowledge  of 
the  Ontogeny  of  the  Lower  Animals. — Stationary  Condition  of  the 
cientific  Study  of  Nature  during  the  Christian  Middle  Ages — 
First  Awakening  of  Ontogeny  in  the  Beginning  of  the  Seventeenth 
Century. — Fabricius  ab  Aqnapendente. — Harvey. — Marcello  Mai- 
ighi.— Importance  of  the  Incubated  Chick. — The  Theories  of  Pre- 
formation  and  Encasement  (Evolution  and  Pre-delineation). — 
Theories  of  Male  and  Female  Encasement. — Either  the  Sperm, 
animal  or  the  Egg  as  the  Pre-forrued  Individual. — Animalculists : 
Leeuwenhoek,  Hartsoeker,  Spallanzani. —  Ovnlists  :  Haller,  Leib- 
nitz, Bonnet. — Victorv  of  the  Theory  of  Evolution  owing  to  the 
uthority  of  Haller  and  Leibnitz.— Caspar  Friedrich  Wolff.— His 
Fate  and  Works. — Tne  Theoria  Generationis. — Re-formation,  or 
Epigenesis. — The  History  of  the  Evolution  of  the  Intestinal  Canal. 
— The  Foundations  of  the  Theory  of  Germ-layers  (Four  Layers, 
Leaves).— The  Metamorphosis  of  Plants.— The  Germs  of  the 
Cellular  Theory.— Wolff's  Monistic  Philosophy  ... 

CHAPTER  III. 

MODERN  ONTOGENY. 
KARL  ERNST  BAER. 

Karl  Ernst  Baer,  the  Principal  Disciple  of  Wolff.— The  Wiirzhnrg  School 
of  Embryologists :  Dollinger,  Pander,  Baer. — Pander's  Theory  of 
Germ-layers. — Its  Full  Development  by  Baer. — The  Disc-shaped 
first  parts  into  Two  Germ-layers,  each  of  which  again  divides  into 
Two  Strata.  The  Skin  or  Flesh-stratum  arises  from  the  Outer  or 
Animal  Germ- layer.  The  Tascular  or  Mucous  Stratum  arises  from 
the  Inner  or  Vegetative  Germ-layer.  The  Significance  of  the 
Germ-layers. — The  Modification  of  the  Layers  into  Tubes.  — Baer's 
Discovery  of  the  Human  Egg,  the  Germ-vesicle,  and  Chorda  Dor- 


CONTENTS.  Vli 

PAOB 

salia. — The  Four  Types  of  Evolution  in  the  Four  Main  Groups  of 
the  Animal  Kingdom. — Baer's  Law  of  the  Type  of  Evolution  and 
the  Degree  of  Perfection. — Explanation  of  this  Law  by  the  Theory 
of  Selection. — Baer's  Successors :  Rathke,  Johannes  Miiller,  Bis- 
choff,  Kolliker.— The  Cell  Theory  :  Schleiden,  Schwann.— Its  Appli- 
cation to  Ontogeny  :  Robert  Remak. — Retrogressions  in  Ontogeny  : 
Beichert  and  His. — Extension  of  the  Domain  of  Ontogeny  :  Darwin  48 

CHAPTER  IV. 

THE    EARLIER   HISTORY   OF   PHTLOGENT. 
JEAN  LAMARCK. 

Phylogeny  before  Darwin. — Origin  of  Species. — Karl  Linnaeus'  Idea  of 
Species,  and  Assent  to  Moses'  Biblical  History  of  Creation. — The 
Deluge. — Palaeontology. — George  Cuvier's  Theory  of  Catastrophes. 
— Repeated  Terrestrial  Revolutions,  and  New  Creations. — Lyell's 
Theory  of  Continuity.— The  Natural  Causes  of  the  Constant  Modi- 
fication  of  the  Earth. — Supernatural  Origin  of  Organisms. — 
Immanuel  Kant's  Dualistic  Philosophy  of  Nature. — Jean  Lamarck. 
— Monistic  Philosophy  of  Nature. — The  Story  of  his  Life. — His 
Philosophic  Zoologique. — First  Scientific  Statement  of  the  Doctrine 
of  Descent. — Modification  of  Organs  by  Practice  and  Habit,  in 
Conjunction  with  Heredity. — Application  of  the  Theory  to  Man. — 
Descent  of  Man  from  the  Ape. — Wolfgang  Goethe. — His  Studies 
in  Natural  Science. — His  Morphology. — His  Studies  of  the 
"  Formation  and  Transformation  of  Organisms." — Goethe's  Theory 
of  the  Tendency  to  Specific  Differences  (Heredity)  and  of  Meta- 
morphosis (Adaptation)  ...  ...  ...  ...  ...  70 


CHAPTER  V. 

MODERN  PHYLOGENY. 
CHARLES  DAKWIN. 

Kolation  of  Modern  to  Earlier  Phylogeny. — Charles  Darwin's  Work  on 
the  Origin  of  Species. — Causes  of  its  Remarkable  Success. — The 
Theory  of  Selection  :  the  Interrelation  of  Hereditary  Transmission 
and  Adaptation  in  the  Struggle  for  Existence". — Darwin's  Life  and 
Voyage  Round  the  World — His  Grandfather,  Erasmus  Darwin. — 
Charles  Darwin's  Study  of  Domestic  Animals  and  Plants. — Com. 


vlii  CONTENTS. 

PAC.B 

parison  of  Artificial  with  Natural  Conditions  of  Breeding. — The 
Struggle  for  Existence. — Necessary  Application  of  the  Theory  of 
Descent  to  Man. — Descent  of  Man  from  the  Ape. —  Thomas  Hux- 
ley.— Karl  Vogt.— Friedrich  Kolle.— The  Pedigrees  in  the  Generelle 
Morphologie  and  the  "  History  of  Creation." — The  Genealogical 
Alternative. — The  Descent  of  Man  from  Apes  deduced  from  the 
Theory  of  Descent. — The  Theory  of  Descent  as  the  Greatest  Induc- 
tive Law  of  Biology. — Foundation  of  this  Induction. — Palaeon- 
tology.— Comparative  Anatomy. — The  Theory  of  Rudimentary 
Organs. — Purposelessness,  or  Dysteleology. — Genealogy  of  the 
Natural  System.— Chorology.— ffikology.— Ontogeny.—  Refutation 
of  the  Dogma  of  Species. — The  "  Monograph  on  the  Chalk 
Spongea ;"  Analytic  Evidence  for  the  Theory  of  Descent  ...  93 

CHAPTER  VI. 

THE  EGG-CELL  AND  THE  AMOEBA. 

The  Egg  of  Man  and  of  other  Animals  is  a  Simple  Cell.— Import  and 
Essential  Principles  of  the  Cell  Theory. — Protoplasm  (Cell-snb- 
stance),  and  the  Nucleus  (Cell-kernel),  as  the  Two  Essential  Con- 
stitnent  Parts  of  every  Genuine  Cell. — The  Undifferentiated  Egg- 
cell,  compared  with  a  highly  Differentiated  Mind-cell  or  Nerve-cell 
of  the  Brain. — The  Cell  as  an  Elementary  Organism,  or  an  Indi- 
vidual of  the  First  Order.— The  Phenomena  of  its  Life.— The 
Special  Constitution  of  the  Egg-cell. — Yelk. — The  Germ-vesicle. — 
The  Germ-spot. — The  Egg-membrane,  or  Chorion. — Application  of 
the  Fundamental  Principle  of  Biogeny  to  the  Egg.cell. — One-celled 
Organisms. — The  Amoebae. — Organization  and  Vital  Phenomena. — 
Their  Movements. — Amoeboid  Cells  in  Many-celled  Organisms. — 
Movements  of  such  Cells,  and  Absorption  of  Solid  Matter. — Absor- 
bent Blood  Corpuscles. — Comparison  of  Amoeba  with  Egg-cell. — 
Amoeboid  Egg-cells  of  Sponges. — The  Amoeba  as  the  Common 
Ancestral  Form  of  Many-celled  Organisms  ...  ...  ...  120 


CHAPTER  VIL 

THE  PROCESSES  OF  EVOLUTION  AND  IMPREGNATION. 

Development  of  the  Many-celled  from  the  One-celled  Organism. — The 
Cell-hermit  and  the  Cell-state.— The  Principles  of  the  Formation 
of  the  State. — The  Differentiation  of  the  Individuals  as  the 


CONTENTS. 


PACK 

Standard  of  Measurement  for  the  Grade  of  the  State. — Parallel 
between  the  Processes  of  Individual  and  of  Race  Development. — 
The  Functions  of  Evolution. — Growth. — Inorganic  and  Organic 
Growth. — Simple  and  Complex  Growth. — Nourishment  and  Change 
of  Substance. — Adaptation  and  Modification. — Reproduction. — 
Asexual  and  Sexual  Reproduction. — Heredity. — Division  of  Labour, 
or  Differentiation. — Atavism,  or  Reversion. — Coalescence. — The 
Functions  of  Evolution  as  yet  very  little  studied  by  Physiology, 
and  hence  the  Evolutionary  Process  has  often  been  misjudged. — 
The  Evolution  of  Consciousness,  and  the  Limits  to  the  Knowledge 
of  Nature. — Fitful  and  Gradual  Evolution. — Fertilization. — Sexual 
Generation. — The  Egg-cell  and  the  Sperm-cell. — Theory  of  the 
Sperm-animals. — Sperm-cells  a  form  of  Whip-cell. — Union  of  the 
Male  Sperm-cell  with  the  Female  Egg-cell.— The  Product  of  this  is 
the  Parent-cell,  or  Cytula. — Nature  of  the  Process  of  Fertilization. 
—  Relation  of  the  Kernel  (Nucleus)  to  this  Process. — Disappear- 
ance of  the  Germ-vesicle. — Monerala. — Reversion  to  the  Monera- 
form.— The  Cytula  ...  ...  ...  ...  ...  ...148 


CHAPTER  VIII. 

EGG-CLEAVAGE  AND  THE  FORMATION  OF  THE   GERM-LAYERS 

First  Processes  after  the  Fertilization  of  the  Egg-cell  is  complete. — 
Original  or  Palingenetic  Form  of  Egg-cleavage. — Significance  of 
the  Cleavage-process. — Mulberry-germ,  or  Morula. — Germ-vesicle, 
or  Blastula  Germ-membrane,  or  Blastoderm. —  Inversion  (In- 
vagination)  of  the  Germ-vesicle. — Formation  of  the  Gastrula. — 
Primitive  Intestine  and  Primitive  Mouth. — The  Two  Primary 
Germ-layers ;  Exoderm  and  Entoderm.— Kenogenetic  Form  of  Egg- 
cleavage. — Unequal  Cleavage  (segmentatio  inequalis)  and  Hood- 
gastrula  (Amphigastrula)  of  Amphibia  and  Mammalia. — Total  and 
Partial  Cleavage. — Holoblastic  and  Meroblastic  Eggs. — Discoidal 
Cleavage  (segmentatio  discoidalis)  and  Disc-gastrula  (Discogastrula) 
of  Fishes,  Reptiles,  Birds. — Superficial  Cleavage  {segmentatio  super- 
ficialis)  and  Vesicular  Gastrula  (Peri-Gastrula)  of  Articulates 
(Arthropoda),— Permanent  Two-layered  Body-form  of  Lower 

Animals. — The  Two-layered   Primaeval  Parent-form  ;    Gastreea. 

Homology  of  the  Two  Primary  Germ-layers  in  all  Intestinal 
Animals  (Metaz'a), — Significance  of  the  Two  Primary  Germ- 
layers. — Origin  and  Significance  of  the  Four  Secondary  Germ- 
layers.— The  Exoderm  or  Skin-layer  gives  rise  to  the  Skin-sensory 


X  CONTENTS. 

PAGI 

Layer  and  the  Skin-fibrous  Layer.—  The  Entodenn  or  Intestinal 
Layer  gives  rise  to  the  Intestinal-fibrous  Layer  and  the  Intestinal- 
glandular  Layer  184 

CHAPTER  IX. 

THE   VERTEBRATE   NATURE   OP  MAN. 

Relation  of  Comparative  Anatomy  to  Classification. — The  Family -rela- 
tionship of  the  Types  of  the  Animal  Kingdom. — Different  Signi- 
ficance and  Unequal  Value  of  the  Seven  Animal  Types. — The 
Gastrcea  Theory,  and  the  Phylogenetic  Classification  of  the  Animal 
Kingdom. — Descent  of  the  Gastraea  from  the  Protozoa. — Descent 
of  Plant-animals  and  Worms  from  the  Gastrasa. — Descent  of 
the  Four  Higher  Classes  of  Animals  from  Worms. — The  Verte- 
brate Nature  of  Man. — Essential  and  Unessential  Parts  of  the 
Vertebral  Organism. — The  Amphioxus,  or  Lancelet,  and  the  Ideal 
Primitive  Vertebrate  in  Longitudinal  and  Transverse  Sections. — 
The  Notochord.— The  Dorsal  Half  and  the  Ventral  Half.— The 
Spinal  Canal.— The  Fleshy  Covering  of  the  Body.— The  Leather- 
skin  (corium). — The  Outer-skin  (epidermis). — Body-cavity  (cceloma). 
—The  Intestinal  Tube.— The  Gill-openings.— The  Lymph- vessels. 
— The  Blood-vessels.— The  Primitive  Kidneys  and  Organs  of  He- 
production. — The  Products  of  the  Four  Secondary  Germ-layers  ...  241 

CHAPTER  X. 

THE  CONSTRUCTION  OF  THE  BODY  FROM  THE  GERM- 
LAYERS. 

The  Original  (Palingenetic)  Development  of  the  Vertebrate  Body  from 
the  Gastrnla. — Relation  of  this  Process  to  the  Later  (Kenogenetic) 
Germination,  as  it  occurs  in  Mammals. — The  most  important  act  in 
the  Formation  of  the  Vertebrate. — The  Primary  Germ-layers,  and 
also  the  Secondary  Germ-layers,  which  arise  by  Fission  of  the  Prima- 
ries, originally  form  Closed  Tabes. — Contemporaneously  with  the 
Completion  of  the  Yelk-sac,  the  Germ-layers  flatten,  and  only  later 
again  assume  a  Tabular  Form. — Origin  of  the  Disc-shaped  Mamma- 
lian Germ-area. — Light  Germ-area  (area  pellucida)  and  Dark  Germ- 
area  (area  opaca). — The  Oval  Germ-shield,  which  afterwards 
assumes  the  Shape  of  the  Sole  of  a  Shoe,  appears  in  the  Centre  of 
the  Light  Germ-area  (a.  pellucida) .— The  Primitive  Streak 


CONTENTS.  XI 

PAGE 

separates  the  Germ-shield  into  a  Eight  and  Left  Half.— Below  the 
Dorsal  Furrow  the  Central  Germ-layer  parts  into  the  Notochord 
and  the  Two  Side-layers. — The  Side-layers  split  horizontally  into 
Two  Layers :  The  Skin-fibrous  Layer  and  the  Intestinal-fibrous 
Layer. — The  Primary  Vertebral  Cords  separate  from  the  Side- 
layers. — The  Skin-sensory  Layer  separates  into  Three  Parts :  the 
Horny  Layer,  Spinal  Canal,  and  Primitive  Kidney. — Formation  of 
the  Coeloin  and  the  First  Arteries. — The  Intestinal  Canal  proceeds 
from  the  Intestinal  Furrow. — The  Embryo  separates  from  the  Germ- 
vesicle. — Around  it  is  formed  the  Amnion-fold,  which  coalesces 
over  the  back  of  the  Embryo,  so  as  to  form  a  Closed  Sac. — The 
Amnion. — The  Amnion-water. — The  Yelk-sac,  or  Navel-vesicle.— 
The  Closing  of  the  Intestinal  and  Ventral  Walls  occasions  the 
Formation  of  the  Navel.— The  Dorsal  and  Ventral  Walls  ...  274 


CHAPTER  XL 

GENERAL  STRUCTURE  AND  ARTICULATION  OF  THE 
INDIVIDUAL.  ' 

Sssential  Agreement  between  the  Chief  Palingenetic  Germ  Processes 
in  the  case  of  Man  and  in  that  of  other  Vertebrates. — The  Human 
Body,like  that  of  all  Higher  Animals,  develops  from  Two  Primary  and 
Four  Secondary  Germ-layers. — The  Skin-sensory  Layer  forms  the 
Horn-plate,  the  Medullary  Tube,  and  the  Primitive  Kidneys. — The 
Middle  Layer  (Mesoderm)  breaks  up  into  the  Central  Notochord, 
the  Two  Primitive  Vertebral  Cords,  and  the  Two  Side-layers.— 
The  latter  split  up  into  the  Skin-fibrous  Layer  and  the  Intestinal- 
fibrous  Layer. — The  Intestinal-glandular  Layer  forms  the  Epi- 
thelium of  the  Intestinal  Canal,  and  of  all  its  Appendages. — Onto- 
genetic  and  Phylogenetio  Fission  of  the  Germ-layers. — Formation 
of  the  Intestinal  Canal. — The  Two-layered  Globular  Intestinal 
Germ-vesicle  of  Mammals  represents  the  Primitive  Intestine.— 
Head  Intestinal  Cavity,  and  Pelvic  Intestinal  Cavity. — Mouth 
Groove  and  Anal  Groove. — Secondary  Formation  of  Mouth  and 
Anus. — Intestinal  Navel  and  Skin-navel. — Movement  of  the 
Primitive  Kidneys  from  the  Outside  to  the  Inside.— Separation  of 
the  Brain  and  Spinal  Marrow. — Rudiments  of  the  Brain-bladders. 
The  Articulation  or  Metameric  Structure  of  the  Body. — The 
Primitive  Vertebrae  (Trunk- Segments,  or  Metamera). — The  Con- 
struction and  Origin  of  ihe  Vertebral  Column. — Vertebral  Bodies 
and  Vertebral  Arches. — Skeleton-plate  and  Muscle-plate. — Forma- 


CONTENTS. 

MOi 

tion  of  the  Skull  from  the  Head-plates. — Gill-openings  and  Gill- 
arches. — Sense-organs. — Limbs. — The  Two  Front  Limbs  and  the 
Two  Hind  Limbs  ...  SL'S 


CHAPTER  XII. 

THE  GERM-MEMBRANES  AND  THE  FIRST  CIRCULATION  OF 
THE  BLOOD. 

The  Mammalian  Organization  of  Man. — Man  has  the  same  Bodily 
Structure  as  all  other  Mammals,  and  his  Embryo  develops  iu 
exactly  the  same  way. — In  its  Later  Stages  the  Human  Embryo  is 
not  essentially  different  from  those  of  the  Higher  Mammals,  and  in 
its  Earlier  Stages  not  even  from  those  of  all  Higher  Vertebrates. — 
The  Law  of  the  Ontogenetic  Connection  of  Systematically  Related 
Forms. — Application  of  this  Law  to  Man. — Form  and  Size  of  the 
Human  Embryo  in  the  First  Four  Weeks. — The  Human  Embryo  iu 
the  First  Month  of  its  Development  is  formed  exactly  like  that  of 
any  other  Mammal. — In  the  Second  Month  the  First  Noticeable 
Differences  .appear. — At  first,  the  Human  Embryo  resembles  those 
of  all  other  Mammals  ;  later,  it  resembles  only  those  of  the  Higher 
Mammals. — The  Appendages  and  Membranes  of  the  Human 
Embryo. — The  Yelk-sac. — The  Allantois  and  the  Placenta. — The 
Amnion. — The  Heart,  the  First  Blood-vessels,  and  the  First  Blood, 
arise  from  the  Intestinal-fibrous  Layer. — The  Heart  separates 
itself  from  the  Wall  of  the  Anterior  Intestine. — The  First 
Circulation  of  the  Blood  in  the  Germ-area  (a.  germinativa)  :  Yelk- 
arteries  and  Yelk-veins. — Second  Embryonic  Circulation  of  the 
Blood,  in  the  Allantois :  Navel-arteries  and  Navel-veins. — Divisions 
of  Human  Germ-history  C<G3 


CHAPTER  XIIL 

THE  STRUCTURE  OF  THE  BODY  OF  THE  AMPHIOXUS  AND 
OF  THE  ASCIDIAN. 

Causal  Significance  of  the  Fundamental  Law  of  Biogeny. — Influence 
of  Shortened  and  Vitiated  Heredity. — Kenogenetic  Modification  of 
Palingenesis.— The  Method  of  Phylogeny  based  on  the  Method  of 
Geology. — Hypothetic  Completion  of  the  Connected  Evolutionary 
Series  by  Apposition  of  the  Actual  Fragments. — Phylogenetio 
Hypotheses  are  Reliable  and  Justified.— Importance  of  the  Amphi- 


CONTEXTS.  xiii 

PACK 

oxns  and  the  Ascidian.— Natural  History  and  Anatomy  of  the 
Ainphioxus. — External  Structure  of  the  .Body. — Skin-covering. — 
Outer-skin  (Epidermis)  and  Leather-skin  (Corium). — Notochord. — 
Medullary  Tube. — Organs  of  Sense. — Intestine  with  an  Anterior 
Respiratory  Portion  (Gill-intestine)  and  a  Posterior  Digestive 
Portion  (Stomach-intestine). — Liver. — Pulsating  Blood-vessels. — 
Dorsal  Vessel  over  the  Intestine  (Gill-vein  and  Aorta). — Ventral 
Vessel  -under  the  Intestine  (Intestinal  Vein  and  Gill-artery). — 
Movement  of  the  Blood. — Lymph-vessels. — Ventral  Canals  and 
Side  Canals  —  Body-cavity  and  Gill-cavity.  —  Gill-covering.  — 
Kidneys.  —  Sexual  Organs. — Testes  and  Ovaries.  —  Vertebrate 
Nature  of  Amphioxns. — Comparison  of  Amphioxus  and  Young 
Lamprey  (Petromyzon) . — Comparison  of  Amphioxns  and  Ascidian. 
— Cellulose  Tunic. — Gill-sac. — Intestine. — Nerve-centres. — Heart. 
— Sexual  Organs  ...  ...  ...  ...  ...  ...  40fi 


CHAPTER  XIV. 

GERM-HISTORY   OF   THE    AMPHIOXUS    AND    OF    THE 
ASCIDIAN. 

Relationship  of  the  Vertebrates  and  Invertebrates. — Fertilization  of  the 
Amphioxus. — The  Egg  undergoes  Total  Cleavage,  and  changes  into 
a  Spherical  Germ-membrane  Vesicle  (Blastula). — From  this  the 
Intestinal  Larva,  or  Gastrula,  originates  by  Inversion. — The 
Gastrnla  of  the  Amphioxus  forms  a  Medullary  Tube  from  a  Dorsal 
Furrow,  and  between  this  and  the  Intestinal  Tube,  a  Notochord : 
on  both  Sides  the  latter  is  a  Series  of  Muscle-plates ;  the  Mate.mera. 
— Fate  of  the  Four  Secondary  Germ-layers. — The  Intestinal  Canal 
divides  into  an  Anterior  Gill-intestine,  and  a  Posterior  Stomach, 
intestine.— Blood-vessels  and  an  Intestinal-muscle  Wall  originate 
from  the  Intestinal-fibrous  Layer. — A  Pair  of  Skin-folds  (Gill, 
roofs)  grow  out  from  the  Side-wall  of  the  Body,  and,  by  Coales- 
cence, form  the  Ventral  Side  of  the  Large  Gill-cavity. — The 
Ontogeny  of  the  Ascidian  is,  at  first,  identical  with  that  of  the 
Amphioxns. — The  same  Gastrula  is  Developed,  which  forms 
a  Notochord  between  the  Medullary  and  Intestinal  Tubes. — 
Retrogressive  Development  of  the  same. — The  Tail  with  the 
Notochord  is  shed. — The  Ascidian  attaches  itself  firmly,  and 
envelops  itself  in  its  Cellulose  Tunic. — Appendicnlaria,  a  Tunicate 
which  remains  throughout  Life  in  the  Stage  of  the  Larval  Ascidian 
and  retains  the  Tail-fin  with  the  Chorda  (Chordonia).— General 
Comparison  and  Significance  of  the  Amphioxus  and  the  Ascidian  43'j 


LIST    OF   PLATES. 


Plate  1.  (Frontinpiece).  Development  of  the  face  in  Mammals 
(Man.  Bat,  Cat,  Sheep)  in  three  different  stages 

Explanation  vol.  ii.     34C 

Plate  II.  (between  p.  240  and  p.  241).  Total  egg-cleavage.  Gas- 
trulation  of  holoblastic  eggs  (primordial  and  unequal  cleavage) 

Explanation     240 

Plate  III.  (between  p.  240  and  p.  241).  Partial  egg-cleavage. 
Gastrulation  of  meroblastic  eggs  (discoidal  and  superficial 
cleavage)  ...  ...  ...  ...  Explanation  240 

Plate  IV.  (between  p.  320  and  p.  321 ).  Diagrammatic  transverse 
section  through  various  ontogenetic  and  phylogenetic  stages 
in  the  development  of  the  human  body,  showing  the  formation 
of  this  from  the  four  secondary  germ-layers  ...  Explanation  321 

Plates  V.  (between  p.  320  and  p.  321).  Diagrammatic  longitu- 
dinal sections  through  various  germ  and  tribal  forms  of  Man, 
showing  their  formation  from  the  four  secondary  germ-layers 

Explanation     323 

Plate  VI.  (between  p.  362  and  p.  363).  Comparison  of  the 
embryos  of  a  Fish,  an  Amphibian,  a  Reptile,  and  a  Bird,  in 
three  different  stages  of  evolution  ...  ...  Explanation  362 

Plate  VII.  (between  p.  362  and  p.  363).  Comparison  of  the 
embryos  of  four  different  Mammals  (Pig,  Ox,  Rabbit,  and 
Man)  in  three  different  stages  of  evolution  ...  Explanation  362 

Plate  VIII.  (between  p.  404  and  p.  405).  Representation  of  two 
human  embryos,  the  one  of  nine,  the  other  of  twelve  weeks  : 
the  latter  within  the  egg-membranes  ...  Explanation  405 

Plate  IX.  (between  p.  404  and  p.  405).  Representation  of  a 
human  embryo  of  five  months,  natural  size,  within  the  egg- 
membranes  ...  ...  ...  ...  Explanation  405 

Plate  X.  (between  p.  438  and  p.  439).  Germ-history  of  Ascidian 
and  Amphioxus  ...  ...  ...  ...  Explanation  436 

Plate  XI.  (between  p.  438  and  p.  439).  Structure  of  the  body 
of  Ascidian,  Amphioxus,  and  larva  of  Petromyzon 

Explanation    437 


LIST  OF  WOODCUTS. 


FTOJBB 

1.  Human  egg-cell 

2.  Human  liver-cell        . 

3.  Epithelium  cell  from  tongue  124 

4.  Thorny  cells  of  epidermis 

5.  Human  bone-cells 

6.  Enamel  cells  of  tooth 

7.  A  mind-cell 

8.  Blood-cells   in   process   of 

division  .         .        •         * 

9.  Active  lymph-cells    . 

10.  Primitive  eggs  of  various 

animals  .... 

11.  Mammalian  egg-cell . 

12.  Egg-cell  of  Hen 

13.  An  Amoeba        .        .        . 

14.  Egg-cell  of  a  Chalk-sponge 

15.  Blood-cells  absorbing  mat- 

tor          .... 

16.  Blood-cells  dividing  . 

17.  Sperm-cells  (seed-cells)     . 

18.  Fertilization  of  mammalian 

egg          .... 

19.  Monerula  of  Mammal 

20.  Moneron  dividing      .         . 
12.  Cytula  of  Mammal    .        . 


PAGE 

122 

FIGURE 

22.  G 

124 

23.  G 

124 

24.  G 

125 

25.  G 

126 

26.  G 

126 

27.  G 

128 

28.  G 

29.  G 

131 

30.  C 

132 

31.  C 

134 

32-35 

136 

86.  It 

139 

37.  C 

142 

38.  B 

144 

39.  E 

40.  E 

145 

41.  G 

159 

42.  E 

173 

43.  G 

44.  E 

175 

45.  E 

179 

46.  ft 

180 

47.  B 

181 

48.  I 

PAOE 

Germination  of  a  Coral      .  190 

Gastrulaof  Gastrophysema  193 

Gastrula  of  Sagitta   .        .  193 

Gastrula  of  Uraster  .        .  193 

Gastrula  of  Nanplius         .  193 

Gastrula  of  Limnseus          .  193 

Gastrula  of  Amphioxus      .  193 

Gastrula  of  Olynthus        .  195 
Cells    of    primary    germ- 
layers      .         .         .         .198 

Cleavage  of  Frog's  egg      .  203 

35.  Gastrulationof  the  Toad  206 

Monerula  of  Eabbit  .        .  210 

Cytula  of  Babbit       .        .  210 

Babbit-egg  with  two  cells  210 

Babbit-egg  with  four  cells  212 

Babbit-egg  with  eight  cells  212 

Gastrula  of  Babbit    .        .  213 

Egg  of  an  Osseous  Fish     .  217 

Gastrnla  of  an  Osseous  Fish  219 

Egg-cell  of  Hen          .        .  223 

Egg-cleavage  of  Bird        .  225 

Mulberry-germ  of  Chick  .  228 

Bladder-germ  of  Chick      .  228 

Invaginated  germ  of  Chick  228 


FIGURE 

49.  Gastrula  of  Chick 

50,  51.  Four  secondary  germ 

layers     . 

52-56.  Diagrammatic  longitu- 
dinal and  transverse  sec- 
tions through  the  ideal 
Primitive  Vertebrate 

57, 58.        „ 

59.  „ 

60.  „ 
61. 

62-C9.  Diagrammatic  trans- 
verse sections  through 
the  mostimportant  germ- 
forms  of  the  ideal  Primi- 
tive Vertebrate 

70.  Diagrammatic     transverse 

sections  through  various 
mammalian  germs  (ex- 
plaining the  separation 
of  the  intestine  from  the 
yelk-sac). 

71.  Gastrula  of  Mammal . 

72.  Intestinal  germ-vesicle  of 

Mammal 

73.  Transverse  section  through 

the    intestinal    germ- 
vesicle  of  Mammal 

74.  Exoderm-cells  of  the  above 
7  5.  Entcderm-cells  of  the  above 
76.  Transverse  section  through 


77-81.  Intestinal  germ-vesicle 

of  Babbit 

82,  83.  Germ-area  of  Rabbit 
84. 
So. 


LIST 

OF   WOODCUTS. 

PACK             FIGURE 

TAOK 

QOQ 

86.  Sole-shaped    germ-shield 

jerm- 

of  Dog. 

298 

. 

236 

87.  Sole-shaped    germ-shield 

gitu- 

of  Chick 

298 

sec- 

88.  Transverse  section 

ideal 

through  germ-shield 

300 

e 

256 

89. 

301 

259 

90. 

302 

263 

91. 

304 

264 

92. 

306 

267 

93. 

309 

rang. 

94.  Development  of  egg-mem- 

ongh 

branes  .... 

312 

erm- 

95.  Transverse  section 

rimi- 

through  germ  of  Chick  . 

317 

. 

276 

96,  97.      „ 

318 

?erse 

98. 

319 

rious 

99. 

331 

(ex- 

100.  Separation  of  the  intes- 

ition 

tine  from  the  yelk-sac 

i  the 

(diagrammatic) 

333 

283 

101.  Longitudinal    section 

288 

through  embryo  Chick  . 

336 

e  of 

102.  Longitudinal   section 

289 

through    head    of    an 

)ugh 

embryo  Chick 

337 

L. 

103-105.  Lyre-shaped     Chick 

289 

embryo 

342 

bove 

290 

106,  107.  Germ-disc  or   germ- 

bove 

290 

area  of  Babbit 

344 

mgh 

108,  109.        „ 

345 

. 

293 

110,  111.  Skeleton  of  Man       . 

351 

side 

112.  Transverse    section 

. 

294 

through  germ  of  Chick  . 

352 

it    . 

296 

113.  Human  neck-vertebra 

854 

297 

114.  Human  chest-vertebra     . 

354 

298 

115.  Human  lumbar-vertebra  . 

354 

LIST   OF   WOODCUTS. 


XVII 


116, 117.  Head      of      embryo 

Chick    .         .         .         .356 

118.  Head  of  embryo  Dog       .     356 

119.  Rudiments  of  the  limbs   .     357 

120.  „  „  359 

121.  Lyre-shaped  germ  of  Dog     367 

122.  Human  germs  from   the 

second  to  the  fifteenth 
week     .        .        .        .368 
123, 124.  Anatomy   of   human 
germs    (four    and    five 
weeks)  .         .         .         .370 

125.  Head  of  Nose -ape    .        .     374 

126.  Head  of  Julia  Pastrana    .     374 
127-131.  Human      eggs     and 

germs   from   second  to 
sixteenth  weeks    .         .     376 
132,133.       „  „  377 

134.  „  „  378 

135.  Chick  germ  with  allantois    380 

136.  Dog  germ  with  allantois  .     381 

137.  „  „  382 

138.  Pregnant    human    uterus 

with     egg  -  membranes 

and  navel-cord      •        .     384 


FIOtTllB  PAOB 

139.  Development  of  egg-mem- 

branes  .         .         .         .  385 

140.  Development  of  amnion  .  387 

141.  „  388 

142.  „                 „  389 
143,144.  Development  of  heart  392 
145,146.        „                „  393 

147.  „                „  395 

148.  First  circulation  of  the 

blood     ....  '396 

149.  „                 „  397 

150.  „                „  398 

151.  Amphioxus  lanceolatns    .  420 

152.  Transverse    section 

through  Amphioxus        .  424 

153.  AnAscidian    .        .        .431 

154.  Another  Ascidian     .        .  434 

155.  Gastrula  of  Amphioxus    .  444 

156.  Gastrnla  of  Sponge          .  445 

157.  Transverse  section 

through  Amphioxu  s  larva    447 

158-160.        „                „  452 

161.  Transverse    section 

through  Vertebrate       .  457 

162.  Appendicularia        .        .  459 


LIST   OF  GENETIC  TABLES. 


TABLE  PAGE 

I.     Systematic  Survey  of  the  main  branches  of  Biogeny     ...       24 
II.     Systematic  Survey  of  the  constituent  parts  of  the  one- 
celled  germ-form  before  and  after  fertilization          ...     183 

III.  Systematic  Survey  of  the  most  important  differences  in 

the  egg-cleavage  and  gastrulation  of  animals  . . .     241 

IV.  Systematic  Survey  of  the  five  first  germinal  stages  of 

animals,  with  reference  to  the  four  main  forms  of 
egg-cleavage      ...  ...  ...  ...  ...     242 

V.  Systematic  Survey  of  some  of  the  most  important  obser- 
vations in  the  rhythm  of  egg-cleavage  ...  ...  243 

VL     Systematic  Survey  of  some  of  the  most  important  organs 
of  the  ideal  Primitive  Vertebrates,  and  their  de- 
velopment from  the  germ-layers   ...  ...  ...     273 

VIL     Systematic  Survey  of  the  development  of  the  human 

organ-systems  from  the  germ-layers  ...  ...     327 

VIII.     Systematic   Survey  of  the  most  important  section  of 

human  germ-history        ...  ...  ...  ...     402 

IX.     Systematic  Survey  of  the  most  important   homologies 
between  the  embryo  of  Man,  and  the  embryo  of 
the  Ascidian  and  the  Amphioxus  in  a  corresponding 
stage  of  development,   on  the  one  hand,  and  the 
developed  Man  on  the  other       ...  ...  ...     465 

X.  Systematic  Survey  of  the  relationship  in  form  of  the 
Ascidian  and  Amphioxus  on  the  one  hand,  of  the  Fish 
and  Man  on  the  other,  in  a  fully  developed  condition  466 

XI      Ontogenetic  cell  pedigree  of  the  Amphioaus    ...  ...    407 


PREFACE  TO  THE  FIRST  EDITION. 


THESE  chapters  on  Anthropogeny  are  the  first  attempt 
to  render  the  facts  of  human  germ-history  accessible  to  a 
wider  circle  of  educated  people,  and  to  explain  these  facts 
by  human  tribal  history.  I  have  not  overlooked  the  great 
difficulty  and  danger  involved  in  thus  entering  for  the 
first  time  on  ground  which  is  so  especially  full  of  risks. 
No  other  branch  of  natural  science  yet  remains  so  ex- 
clusively confined  to  its  own  technical  students ;  no  other 
branch  has  been  so  wilfully  obscured  and  mystified,  by 
priestly  influence,  as  has  the  germ-history  of  Man.  If, 
even  now,  we  say  that  each  human  individual  develops 
from  an  egg,  the  only  answer,  even  of  most  so-called  edu- 
cated men,  will  be  an  incredulous  smile  ;  if  we  show  them 
the  series  of  embryonic  forms  developed  from  this  human 
egg,  their  doubt  will,  as  a  rule,  change  into  disgust.  Few 
educated  men  have  any  suspicion  of  the  fact,  that  these 
human  embryos  conceal  a  greater  wealth  of  important 
truths,  and  form  a  more  abundant  source  of  knowledge  than 
is  afforded  by  the  whole  mass  of  most  other  sciences  and 
of  all  so-called  "revelations." 


XX  PREFACE   TO   THE    FIRST    EDITION. 

Nor  is  this  surprising,  when  we  see  what  a  little  way 
the  knowledge  of  human  evolution  has  spread  even  among 
the  very  students  of  Nature.  Even  in  most  works  devoted 
to  the  Natural  History,  Anatomy,  Physiology,  Ethnology, 
and  Psychology  of  Man,  it  is  evident  at  a  glance  that  their 
authors,  if  not  ignorant,  have  at  least  a  very  superficial 
knowledge  of  human  germ-history,  and  that  tribal  history 
lies  far  beyond  them.  The  name  of  Darwin  is,  indeed,  in 
every  mouth.  But  few  persons  have  really  assimilated 
the  theory  of  descent,  as  reformed  by  him  ;  few  have  made 
it  part  of  themselves.  To  show  how  far  even  biologists  of 
repute  are  from  thoroughly  understanding  the  history  of 
evolution,  no  more  remarkable  recent  instance  can  be 
found  than  the  well-known  address,  on  "The  Limits  of 
Natural  Knowledge,"  delivered  by  the  celebrated  physio- 
logist, Du  Bois  Eeymond,  in  1873,  before  the  naturalisto 
assembled  at  Leipzig.  This  eloquent  address,  the  source 
of  such  triumph  to  the  opponents  of  the  theory  of  evolu- 
tion, the  cause  of  such  pain  to  all  friends  of  intellectual 
advance,  is  essentially  a  great  denial  of  the  history  of 
evolution.  No  thoughtful  naturalist  will  disagree  with  the 
Berlin  physiologist  when,  in  the  first  half  of  his  address, 
he  explains  the  limits  of  natural  knowledge,  as  they  are  at 
present  set  to  man  by  his  vertebrate  nature.  But  it  is 
equally  certain  that  every  monistic  naturalist  will  protest 
against  the  second  half  of  the  address,  in  which,  not  only 
is  another  limit,  assumed  to  be  different  (but  in  reality 
identical),  indicated  for  human  knowledge,  but  the  con- 
clusion is  finally  drawn,  that  man  will  never  pass  over 
these  limits  :  "  We  shall  never  know  that !  Ignorabimus! " 

As  the  unanimous  thanks  of  the  Ecclesia  militans  have 


PREFACE   TO   THE   FIRST    EDITION.  XXi 

been  gained  by  the  author  of  this  "  Ignorabimus,"  the  most 
deserving  student  of  the  electricity  of  nerves  and  muscles, 
we  must  here  most  emphatically  protest  in  the  name  of 
advancing  natural  knowledge  and  of  all  science  capable 
of  development.  Had  our  one-celled  Amoeba-ancestors  of 
the  Laurentian  Period  been  told  that  their  descendants 
would  afterwards,  in  the  Cambrian  Period,  produce  a  many- 
celled  Worm-like  organism  possessed  of  skin  and  intestine, 
muscles  and  nerves,  kidneys  and  blood-vessels,  they  would 
certainly  not  have  believed ;  nor,  again,  would  these  Worms 
have  believed,  had  they  been  told  that  their  descendants 
would  develop  into  skull-less  Vertebrates,  such  as  the 
Amphioxus ;  nor  would  these  Skull-less  Animals  have 
credited  that  their  posterity  would  ever  become  Skulled 
Animals  (Craniota).  Our  Silurian  Primitive-fish  ancestors 
would  have  been  equally  hard  to  convince  that  their  off- 
spring of  the  Devonian  Period  would  acquire  amphibian 
form,  and  yet  later,  in  the  Triassic  Period,  would  appear 
as  Mammals ;  the  latter,  again,  would  have  deemed  it  im- 
possible that,  in  Tertiary  times,  a  very  late  descendant 
of  theirs  would  acquire  human  form,  and  would  gather  the 
splendid  fruits  of  the  tree  of  knowledge.  All  these  would 
have  answered :  "  We  shall  never  change,  nor  shall  we 
ever  understand  the  history  of  our  evolution!  Nunquam 
mutabimur  !  Semper  ignordbimus  !  " 

With  this  Ignorabimus  the  Berlin  school  of  Biology 
tries  to  stop  science  in  its  advance  along  the  paths  of 
evolution.  This  seemingly  humble  but  really  audacious 
"Ignorabimus"  is  the  "  Ignoratis"  of  the  infallible 
Vatican  and  of  the  "  black  international  "  which  it  leads  ; 
that  mischievous  host,  against  which  the  modern  civilized 


XX11  PREFACE   TO   THE   FIRST   EDITION. 

state  has  now  at  last  begun  in  earnest  the  "struggle 
for  culture."  In  this  spiritual  warfare,  which  now  moves 
all  thinking  humanity,  and  which  prepares  the  way  for  a 
future  existence  more  worthy  of  man,  spiritual  freedom 
and  truth,  reason  and  culture,  evolution  and  progress 
stand  on  the  one  side,  marshalled  under  the  bright  banner 
of  science ;  on  the  other  side,  marshalled  under  the  black 
flag  of  hierarchy,  stand  spiritual  servitude  and  falsehood, 
want  of  reason  and  barbarism,  superstition  and  retrogres- 
sion. The  trumpet  of  this  gigantic  spiritual  warfare 
marks  the  dawn  of  a  new  day  and  the  end  of  the  long 
darkness  of  the  Middle  Ages.  For  modern  civilization,  in 
spite  of  the  progress  of  culture,  lies  bound  in  the  fetters 
of  the  hierarchy  of  the  Middle  Ages ;  and  social  and  civil 
life  is  ruled,  not  by  the  science  of  truth,  but  by  the  faith 
of  the  church.  We  need  but  mention  the  mighty  influence 
which  irrational  dogmas  still  exercise  on  the  elementary 
education  of  our  youth;  we  need  but  mention  that  the 
state  yet  permits  the  existence  of  cloisters  and  of  celibacy, 
the  most  immoral  and  baneful  ordinances  of  the  "  only- 
saving  "  church ;  we  need  but  mention  that  the  civilized 
state  yet  divides  the  most  important  parts  of  the  civil 
year  in  accordance  with  church  festivals ;  that  in  many 
countries  it  allows  public  order  to  be  disturbed  by  church 
processions,  and  so  on.  We  do  indeed  now  enjoy  the 
unusual  pleasure  of  seeing  "most  Christian  bishops"  and 
Jesuits  exiled  and  imprisoned  for  their  disobedience  to  the 
laws  of  the  state.  But  this  same  state,  till  very  recently, 
harboured  and  cherished  these  most  dangerous  enemies  of 
reason. 

In  this  mighty  "  war  of  culture,"  affecting  as  it  does 


PREFACE  TO  THE   FIRST   EDITION. 

the  whole  history  of  the  World,  and  in  which  we  may  well 
deem  it  an  honour  to  take  part,  no  better  ally  than  Anthro- 
pogeny  can,  it  seems  to  me,  be  brought  to  the  assistance 
of  struggling  truth.  The  history  of  evolution  is  the  heavy 
artillery  in  the  struggle  for  truth.  Whole  ranks  of  dualistic 
sophisms  fall  before  the  monistic  philosophy,  as  before  the 
chain  shot  of  artillery,  and  the  proud  structure  of  the 
Roman  hierarchy,  that  mighty  stronghold  of  infallible 
dogmatism,  falls  like  a  house  of  cards.  Whole  libraries 
of  church  wisdom  and  false  philosophy  melt  away  as  soon 
as  they  are  seen  in  the  light  afforded  by  the  history  of 
evolution.  The  church  militant  itself  furnishes  the  most 
striking  evidences  of  this,  for  it  never  ceases  to  give  the 
lie  to  the  plain  facts  of  human  germ-history,  condemning 
them  as  "diabolical  inventions  of  materialism."  In  so 
doing  it  gives  the  most  brilliant  witness  that  it  recognizes 
as  unavoidable  the  conclusions  which  we  have  drawn  from 
these  facts  as  to  tribal  history,  as  to  the  true  causes  of 
these  facts. 

In  order  to  render  these  little  known  facts  of  germ- 
history  and  their  causal  explanation  by  tribal  history 
accessible  to  as  wide  a  circle  of  educated  readers  as  pos- 
sible, I  have  followed  the  same  course  as  that  which 
I  adopted,  six  years  ago,  in  my  "Natural  History  of 
Creation,"  of  which  the  "  Anthropogeny  "  forms  a  second, 
more  detailed  part.  In  the  summer  of  1873  I  had  the 
academical  lectures,  on  the  outlines  of  the  history  of 
human  evolution,  which  I  have  delivered  during  the  last 
twelve  years  in  Jena  before  a  mixed  audience  of  students 
of  all  faculties,  taken  down  in  shorthand  by  two  of  that 
nudience,  Messrs.  Kiessling  and  Schlawe.  The  task  I 


XXiV  PREFACE   TO   THE   FIRST   EDITION. 

undertook  in  publishing  these  was  indeed  much  harder  than 
that  incurred  in  the  "  Natural  History  of  Creation;  "  for 
while  the  latter  passed  lightly  through  the  widest  circle 
of  biological  phenomena,  and  touched  only  on  the  most 
interesting  points,  I  was  obliged,  in  the  "  History  of  the 
Evolution  of  Man,"  to  exhibit  a  much  more  limited  series 
of  phenomena  in  their  proper  connection,  of  which,  indeed, 
each  individual  one  is  interesting  in  its  proper  place, 
although  they  are  of  very  various  degrees  of  interest. 
Moreover,  the  comprehension  of  form -phenomena,  with 
which  human  germ- history  deals,  is  among  the  most 
difficult  of  morphological  tasks ;  the  academical  lectures 
on  the  history  of  human  evolution  are  rightly  considered 
even  by  medical  men,  who  are  previously  acquainted  with 
the  anatomical  features  of  the  human  body,  as  the  most 
difficult  to  understand.  I  saw,  therefore,  that,  if  I  desired 
to  make  the  road  into  this  dark  region,  entirely  closed  as 
yet  to  most  men,  really  accessible  to  the  educated  laity, 
I  must,  on  the  one  hand,  limit  myself  as  far  as  possible  in 
my  selection  from  the  abundance  of  empiric  matter,  and 
yet,  on  the  other  hand,  that  I  must  be  careful  not  to  pass 
entirely  over  any  essential  part  of  this  matter. 

Although,  therefore,  I  have  throughout  taken  pains  to 
present  the  scientific  problem  of  Anthropogeny  in  as 
popular  a  form  as  possible,  I  do  not  imagine  that  I  have 
completely  accomplished  this  very  difficult  task.  I  shall, 
however,  have  gained  my  object  if  I  succeed  in  affording 
educated  persons  an  approximate  conception  of  the  most 
essential  outlines  of  human  germ-history,  and  in  con- 
vincing them  that  the  sole  explanation  and  comprehension 
of  the  matter  is  afforded  by  the  corresponding  tribal 


PREFACE   TO   THE    FIRST    EDITION.  XXV 

history.  Perhaps,  at  the  same  time,  I  may  hope  to  con- 
vince some  of  those  specialists,  who  deal  indeed  daily 
with  the  facts  of  germ-history,  but  who  neither  know  nor 
wish  to  know  anything  about  the  true  causes  of  these, 
which  lie  hid  in  tribal  history.  As  this  is  quite  the  first 
attempt  to  present  the  Ontogeny  and  Phylogeny  of  man  in 
their  whole  causal  connection,  I  fear  that,  at  best,  the 
point  at  which  I  aim  lies  far  beyond  the  point  gained. 
But  by  this  each  thinking  man  will,  it  is  to  be  hoped,  be 
convinced  that  only  by  recognizing  this  connection  does 
the  history  of  human  evolution  become  a  science.  On- 
togeny can  only  be  really  understood  through  Phylogeny. 
The  history  of  the  tribe  lays  bare  the  true  causes  of  the 
history  of  the  germ. 

ERNST  HEINKICH  HAECKEL. 

Jena,  July  13,  1874. 


PREFACE  TO   THE  THIRD  EDITION. 


WHEN,  two  years  ago,  I  published  the  first  edition  of  the 
"History  of  the  Evolution  of  Man,"  and  this  was  followed, 
a  few  months  later,  by  an  unaltered  second  edition,  I  was 
fully  conscious  of  the  hazard  involved  in  so  doing,  and 
was  prepared  to  meet  with  numerous  attacks.  These  were 
not  slow  to  come;  and  if  I  were  now  obliged  to  answer  all 
my  opponents,  this  third  edition  might  easily  be  doubled  in 
size.  I  think,  however,  that  I  may  satisfy  myself  with  but 
a  few  remarks. 

The  great  majority  of  my  opponents  are  determined 
enemies  of  the  Doctrine  of  Descent,  who  altogether  deny 
a  natural  evolution  of  organic  nature,  and  who  can 
only  explain  both  the  origin  of  man  and  that  of  animal 
and  plant  species  with  the  help  ot  miracles,  by  super- 
natural creative  acts.  These  adherents  of  the  Creation 
Theory  I  need  not  answer ;  for  Anthropogeny,  as  the 
special  application  of  the  Theory  of  Descent  to  Man, 
naturally  starts  from  the  recognition  of  this  latter  theory : 
ten  years  ago,  in  my  Generelle  Morphologic,  and  again  in 
the  "  Natural  History  of  Creation,"  I  explained  my  own 
conception  of  this  in  sufficient  detail. 


XXviii  PREFACE    TO    THE  THIRD   EDITION. 

I  cannot,  however,  refrain  from  defending  my  stand- 
point against  those  naturalists,  who,  taking  their  position 
indeed  on  the  Theory  of  Descent  and  on  Darwinism,  yet 
combat  my  individual  conception  of  this,  and,  especially, 
regard  my  application  of  the  theory  to  Anthropogeny  as 
erroneous.  Many  of  these  naturalists,  who  were  formerly 
determined  opponents  of  the  Theory  of  Descent,  have 
recently  passed  over  to  Darwin's  camp,  merely  in  order 
not  to  stand  entirely  inactive  at  the  barren  standpoint 
offered  by  negation.  Against  two  of  these  false  Darwinists, 
Wilhelm  His  and  Alexander  Goette,  I  have  defended 
myself  in  a  special  work  on  "  The  Aims  and  Methods  of  the 
Modern  History  of  Evolution"  ("  Ziele  und  Wege  derHeuti- 
gen  Entwickelungsgeschichte."  Jena,  1875).  To  that  work 
I  now  refer.  On  the  other  hand,  I  have  been  forcibly 
attacked  by  naturalists  who  are  really  esteemed  as  well- 
known  and  convinced  adherents  of  the  Theory  of  Evolu- 
tion. Of  these,  Karl  Vogt  and  Albert  Kolliker  require  a  few 
words  of  answer. 

Vogt,  whose  many  services  in  furthering  Zoology  I  have 
always  most  readily  acknowledged,  ranked  second  to  Huxley 
among  those  naturalists  who,  but  a  few  years  after  the 
appearance  of  Darwin's  "Origin  of  Species,"  attempted  to 
apply  the  theory  contained  in  that  work  to  Man  and 
represented  this  as  necessary.  He  afterwards,  however, 
made  no  further  progress  in  the  same  direction.  While,  as 
I  am  convinced,  the  mass  of  facts  already  accumulated  in 
Comparative  Anatomy,  Ontogeny,  Palaeontology,  and  Sys- 
tematic Zoology,  is  amply  sufficient  to  afford  the  most 
general  points  on  which  to  base  the  hypothetic  human 
pedigree,  Karl  Vogt  now  holds  opposed  views,  and  entirely 


PREFACE   TO   THE   THIRD    EDITION. 

rejects  the  ancestral  series  as  I  have  arranged  it.  He 
says :  "  We  have  been  able  to  prove  the  assertion  that  Men 
and  Apes  must  have  originated  from  a  common  line ; — more 
than  this  we  have  never  asserted,  and  further  back  than 
this  it  is  absolutely  impossible  to  prove  anything  or  even 
to  show  with  any  degree  of  probability  more  than  that, 
at  farthest,  the  higher  Mammals  may  perhaps  have  de- 
veloped from  Pouched  Animals  (Marsupialia)."  Against 
this  view  of  Vogt's,  I  assert,  that  with  the  same  logical 
"  certainty  or  probability "  the  common  descent  of  all 
Mammals  from  lower  Vertebrates,  primarily  from  Am- 
phibia, less  immediately  from  Fishes,  may  be  "  proved." 
With  the  same  "  certainty  or  probability  " — I  assert  again 
— the  descent  of  all  these  Skulled  Animals  (Craniota)  from 
Skull-less  forms  (Acrania,  allies  of  Amphioxus),the  descent 
of  these  latter  from  Chorda  Animals  (Chordonia,  forms 
allied  to  Ascidia),  and  the  descent  of  these  Chorda  Animals 
from  low  Worms,  "may  be  proved."  With  the  same 
"  certainty  or  probability  " — I  say  finally — "  we  have  been 
able  to  prove  the  assertion,"  that  these  Worms  must, 
in  their  turn,  have  originated  from  a  Gastrsea  (resembling 
the  gastrula),  and  these  Gastrseads  from  a  one-celled 
organism  (resembling  the  undifferentiated  Amoeba).  Proofs, 
as  I  believe,  of  these  assertions  are  given  in  Chapters 
XIII.-XXV.  of  this  edition. 

The  whole  of  this  hypothetic  pedigree  Karl  Vogt  entirely 
rejects,  without,  however,  substituting  another.  He  espe- 
cially denies  our  relationship  with  the  Selachii  and  the 
Amphioxus,  with  the  Ascidia  and  the  Gastraea,  although  the 
especially  great  phylogenetic  significance  of  these  instruc- 
tive animal-forms  is  almost  unanimously  recognized  by  the 


XXX  PREFACE  TO   THE  THIRD   EDITION. 

first  authorities  in  our  science.  Whilst  Vogt  completely 
opposes  himself  to  these  important  views,  which  from 
day  to  day  become  more  firmly  established,  he  refers  to 
Karl  Semper,  a  "gifted"  naturalist,  who  shares  these 
views  of  Vogt's,  and  who  derives  Vertebrates  from  Einged 
Worms  (Annelida).  I  regret  that  I  can  make  no  use  of 
this  reference;  nor  do  I  find  reason  to  answer  Semper's 
polemic  on  "Haeckelism  in  Zoology"  (" Haeckelismus  in 
der  Zoologie."  Hamburg,  1876) ;  for,  apart  from  his  de- 
fective education  and  his  insufficient  acquaintance  with  the 
whole  subject  of  Zoology,  this  "gifted"  zoologist  is  so 
much  at  variance  with  logic,  as  also  with  truth,  that 
refutation  seems  superfluous.  (Cf.  vol.  i.  p.  91  and  p.  426.) 
An  example  is  sufficient  to  show  this :  In  order  to  indicate 
the  scientific  value  of  "Haeckelism,"  and  in  order  "to 
show  that  this  tendency  must  continually  diverge  more 
and  more  widely  from  the  really  scientific  study  of 
nature,"  Semper  brings  forward  the  fact  that,  "  according 
to  Haeckel's  own  statement,  Darwinism  should  be  the 
religion  of  every  naturalist."  This  last  statement,  which 
I  consider  absurd,  is  not  mine,  but  that  of  my  determined 
opponent,.  Professor  Rutimeyer,  and  I  quoted  the  sentence 
in  the  preface  to  the  third  edition  of  the  "  Natural  History 
of  Creation  "  merely  to  show  the  singular  ground  occupied 
by  its  author. 

The  wide  cleft  which  separates  my  standpoint  of  the 
history  of  evolution  and  of  natural  science,  as  a  whole, 
from  that  of  Vogt  and  Semper  cannot  be  better  indicated 
than  by  our  mutual  position  towards  philosophy.  Karl 
Vogt,  like  his  friend  Karl  Semper,  was  a  sworn  contemner 
of  all  philosophy.  The  former  seizes  every  opportunity  to 


PKEFACE   TO   THE   THIRD    EDITION.  XXXI 

mock  at  philosophic  tendencies  and  researches ;  and  the 
latter  knows  no  more  severe  charge  to  bring  against  me 
than  that  I  seek  to  unite  empiricism  and  philosophy, 
experience  and  idea,  "  observation  and  reflection."  I  am 
certainly  firmly  convinced  that  a  really  scientific  study  of 
nature  can  no  more  dispense  with  philosophic  reflection, 
than  can  healthy  philosophy  ignore  the  results  of  natural 
scientific  experience.  "An  exact  empiricism,"  without 
those  philosophic  thoughts  which  combine  and  explain  the 
raw  material  of  facts,  merely  results  in  the  accumulation 
of  a  lifeless  store  of  knowledge ;  on  the  other  hand, 
"  speculative  philosophy  "  which  knows  nothing  of  the  firm 
basis  afforded  by  natural  scientific  observation,  can  only 
produce  transient  cloud-pictures.  The  most  intimate  com- 
bination and  blending  of  empiricism  and  philosophy  can 
alone  enable  us  to  construct  a  permanent  and  sure  scientific 
structure.  I  still  hold  as  decidedly  as  ever  the  much- 
abused  views  which  I  expressed,  ten  years  ago,  about  this 
matter  in  my  Generelle  Morphologic,  and  the  fundamental 
ideas  which  I  have  here  reproduced. 

Moreover,  he  must  be  very  one-sided  or  short-sighted 
who  does  not  recognize  the  natural  approximation,  which 
is  now  becoming  more  close  in  all  branches  of  human 
knowledge,  between  experimental  and  reflective  study.  The 
enormous  enlargement  of  the  field  of  empiric  knowledge 
which  has  been  brought  about  by  the  progress  of  the  last 
half-century,  has  resulted  in  a  corresponding  specialization 
of  separate  researches,  and  consequently  in  an  isolation  of 
diverging  aims  which  cannot  possibly  continue  to  satisfy. 
All  thoughtful  observers  feel,  more  acutely  in  consequence 
of  this,  that  they  must  raise  themselves  from  the  wearisome 


XXxil  PREFACE   TO   THE   THIRD    EDITION. 

task  of  accumulating  dry  details  to  wider  views,  and  thus  to 
gain  sympathy  with  allied  aims.  On  the  other  side,  the 
sterility  of  such  pure  speculative  philosophy  as  ignores  all 
those  enormous  advances  in  empiric  knowledge,  has  so 
forced  its  way  into  the  consciousness  of  all  sound  thinkers, 
that  they  earnestly  desire  to  fall  back  on  the  firm  basis 
afforded  by  experimental  science. 

The  ever-increasing  flood  of  writings  on  natural  philo- 
sophy, and  essays  on  the  relation  of  philosophy  to  natural 
science,  plainly  indicates  the  happy  growth  of  this  scientific 
unitary  tendency.  Nothing  is  more  favourable  to  this, 
nothing  better  advances  the  combination  of  the  various 
scientific  lines,  than  the  new  theory  of  evolution.  The 
extraordinary  importance  ascribed  to  this  theory,  rests 
especially  on  the  fact  that  it  supplies  a  philosophic  central 
point,  and  just  for  this  very  reason  it  has  in  so  short  a 
time  gained  the  active  interest  of  all  thoughtful  minds. 
It  raises  us  from  a  knowledge  of  facts  to  a  knowledge  of 
causes,  and  thus  affords  a  deeper  satisfaction  to  the 
demand  for  causality  innate  in  human  reason  than  a  mere 
experimental  science  could  ever  supply.  When,  therefore, 
Karl  Vogt  and  many  other  naturalists  entirely  reject  philo- 
sophy, and  will  not  allow  that  it  has  any  point  of  union 
with  what  is  called  "  exact "  natural  science — they  volun- 
tarily renounce  all  the  higher  aims  of  investigation. 
(Cf.  vol.  ii.  p.  387.) 

Albert  Kolliker  occupies  a  similarly  one-sided  stand- 
point. This  author,  in  the  second  edition  of  his  "  History 
of  the  Evolution  of  Man  and  the  Higher  Animals"  ("Ent- 
wickelungsgeschichte  des  Menschen  und  der  Hoheren 
Thiere,"  1876),  in  especially  attacking  the  fundamenlal  law 


PREFACE   TO   THE   THIRD    EDITION. 

of  Biogeny,  has  impugned  the  very  foundation  on  which 
Anthropogeny  rests.  Most  of  his  objections  are,  it  appears 
to  me,  refuted  by  the  explanations  which  I  have  given 
in  this  third  edition  as  to  the  very  important  relations 
of  Palingenesis  and  Kenogenesis.  (Compare  especially 
Chapters  L,  VIII.,  and  X.)  Kolliker  will  not  recognize  the 
Gastrsea  Theory  because  he  has  been  unable  to  discover  a 
gastrula  in  Mammals  and  Birds.  But  his  experiences  are 
opposed  to  the  most  recent  researches  of  Van  Beneden  and 
Rauber,  of  whom  the  former  in  the  case  of  the  Eabbit,  the 
latter  in  the  case  of  the  Chick,  describes  a  kenogenetic 
gastrula-form,  which,  in  accordance  with  the  Gastrsea 
theory,  may  easily  be  referred  to  the  palingenetic  gastrula 
of  the  Amphioxus.  Kolliker  says  finally :  "As  the  last  and 
most  important  argument,  I  bring  forward  the  fact  that 
Phylogeny  as  read  by  Darwin  and  Haeckel  does  not,  it 
appears  to  me,  represent  the  truth."  This  "  most  im- 
portant argument "  is  a  simple  petitio  principii.  The  sen- 
tence might  as  well  be,  "  phylogeny  is  not  true  because  it 
does  not  represent  the  truth." 

How  very  different  in  other  respects  Kolliker's  concep- 
tion of  the  history  of  evolution  is  from  mine  is  most  clearly 
indicated  in  the  "  General  Observations  "  (§  29)  at  the  end 
of  his  book.  The  learned  Wiirzburg  anatomist  there 
explains  with  reference  to  germ -history,  his  "  essential 
agreement  in  fundamental  conceptions "  with  the  un- 
learned Leipzig  anatomist  Wilhelm  His.  I  have  explained 
the  nature  of  these  "  mechanical  fundamental  conceptions  " 
in  Chapter  XXIV.  of  this  book  (vol.  ii.  p.  352),  and  in 
greater  detail  in  my  work  on  "The  Aims  and  Methods  of 
the  Modern  History  of  Evolution  "  ("  Ziele  und  Wege  der 

3 


PREFACE  TO  THE  THIRD   EDITION. 

Heutigen  Entwickelungsgeschichte ").  The  celebrated 
theories  of  His,  of  which  I  have  spoken  as  the  "  envelope 
theory,"  "gum-pouch  theory,"  "waste-rag  theory,"  etc., 
are  the  brilliant  results  of  that  "  gifted "  author's  efforts 
and  mathematical  calculations.  And  yet  many  have 
allowed  themselves  to  be  dazzled  by  the  "  exact "  appear- 
ance of  his  mathematical  formula  The  history  of  the 
evolution  of  organisms,  equally  with  the  history  of  human 
civilization,  can  never  be  the  subject  of  "  exact "  investi- 
gation. The  history  of  evolution  is  in  its  very  nature  an 
historic  natural  science,  as  is  geology.  To  regard  and 
treat  these  and  other  historic  natural  sciences  as  "  exact  " 
leads  to  the  greatest  errors.  This  is  as  true  of  germ- 
history  (Ontogeny)  as  of  tribal  history  (Phylogeny) ;  foi 
between  the  two  there  is  the  most  intimate  causal 
connection. 

Many  naturalists  have  especially  blamed  the  diagram- 
matic figures  given  in  the  Anthropogeny.  Certain  tech- 
nical embryologists  have  brought  most  severe  accusations 
against  me  on  this  account,  and  have  advised  me  to  substi- 
tute a  larger  number  of  elaborated  figures,  as  accurate  as 
possible.  I,  however,  consider  that  diagrams  are  much 
more  instructive  than  such  figures,  especially  in  popular 
scientific  works.  For  each  simple  diagrammatic  figure 
gives  only  those  essential  form-features  which  it  is  intended 
to  explain,  and  omits  all  those  unessential  details  which  in 
finished,  exact  figures,  generally  rather  disturb  and  confuse 
than  instruct  and  explain.  The  more  complex  are  the 
form-features,  the  more  do  simple  diagrams  help  to  make 
them  intelligible.  For  this  reason,  the  few  diagrammatic 
figures,  simple  and  rough  as  they  were,  with  which  Baer 


PREFACE   TO   THE   THIRD    EDITION.  XXXV 

half  a  century  ago  accompanied  his  well-known  "  History 
of  the  Evolution  of  Animals,"  have  been  more  serviceable 
in  rendering  the  matter  intelligible  than  all  the  numerous 
and  very  careful  figures,  elaborated  with  the  aid  of 
camera  lucida,  which  now  adorn  the  splendid  and  costly 
atlases  of  His,  Goette,  and  others.  If  it  is  said  that  my 
diagrammatic  figures  are  "inaccurate,"  and  a  charge  of 
"  falsifying  science  "  is  brought  against  me,  this  is  equally 
true  of  all  the  very  numerous  diagrams  which  are  daily  used 
in  teaching.  All  diagrammatic  figures  are  "inaccurate." 

The  important  advances  in  many  different  directions 
made  daring  the  last  two  years,  both  by  germ-history  and 
tribal  history,  especially  the  reconstruction  of  the  germ- 
layer  theory  and  the  development  of  the  Gastraea  theory, 
have  compelled  me  essentially  to  modify  the  second  and 
third  sections  of  the  Anthropogeny.  Chapters  VIII.,  IX., 
XVI.,  and  XIX.  especially  appear  in  a  new  form ;  but  even 
in  Sections  I.  and  IX.  I  have  been  compelled  to  modify 
much  and  to  improve  many  parts.  At  the  same  time  I 
have  exerted  myself  to  the  utmost,  by  improving  the  formal 
exposition,  to  render  the  extremely  dry  and  unacceptable 
matter  more  interesting.  This  is,  of  course,  an  unusally 
hard  task,  and  I  am  well  aware  how  far  even  this  third 
edition,  in  spite  of  all  my  efforts,  is  from  affording  a  really 
popularly  intelligible  explanation  of  the  Ontogeny  and 
Phylogeny  of  Man.  Because  the  defective  natural  scien- 
tific instruction  in  our  schools,  even  in  the  present  day, 
leaves  educated  men  quite  or  nearly  ignorant  of  the  struc- 
ture and  arrangement  of  their  bodies,  the  anatomical  and 
physiological  foundation  is  usually  wanting,  on  which  alone 
a  true  knowledge  of  human  germ-history,  and  consequently 


PREFACE   TO   TIIE   THIRD   EDITION. 

of  human  tribal  history,  can  be  based.  And  yet,  as  Baer 
says,  "no  investigation  is  more  worthy  of  a  free  and 
thoughtful  man  than  the  study  of  himself."  (Cf.  vol.  i. 
p.  244.)  Hoping,  as  I  do,  that  I  may  have  aided  to  some 
extent  to  bring  about  this  true  self-knowledge,  I  shall  have 
gained  my  purpose  if  my  labours  arouse  an  active  interest 
in  wider  circles  in  the  historic  evolution  of  our  animal 
organism,  and  if  they  advance  the  knowledge  of  this  most 
significant  process. 

ERNST  HEINRICH  HAECKEL. 

Jena,  October  6,  1876. 


PROMETHEUS. 

ENVEIL  thine  heaven,  Zeus,  with  vaporous  cloud. 

And  practise,  like  a  boy  beheading  thistles, 

On  oaks  and  mountain  summits ; 

Yet  must  thou  let  my  earth  alone  to  stand, 

And  these  my  dwellings,  which  thou  didst  not  bniUlj 

And  these  my  flocks,  for  whose  bright  glow 

Thou  enviest  me. 

I  know  not  aught  more  wretched 

Beneath  the  sun  than  you,  ye  Gods  ! 

Who  nourish  piteously, 

With  tax  of  sacrifice  and  reek  of  prayer ;  your  glory 

Would  starve,  if  children  were  not  yet,  and  suppliants, 

So  full  of  hope — and  fools. 

When  I  was  young,  and  knew  not  whence  nor  whither, 

I  used  to  turn  my  dazzled  eyes  to  the  sun, 

As  if  above  me  were 

An  ear  to  listen  to  my  crying, 

A  heart,  like  mine,  to  pity  those  oppress'd. 

Who  aided  me  against  the  Titans'  arrogance  ? 

Who  rescued  me  from  death,  from  slavery  ? 

'Tis  thou  alone  hast  wrought  it  all,  thou  holy,  glowing  heart. 

Thou  didst  glow  young  and  fresh,  though  cheated ;  thanks  for 

saving 
That  slumbering  one  above. 

Why  should  I  honour  thee  ? 

Hast  thou  e'er  lighten 'd  the  woes  of  the  laden  ones  ? 

Hast  thou  e'er  dried  the  tears  of  the  sorrowful? 

[t  was  not  thou  who  welded  me  to  manhood, 

But  Time  the  almighty,  Fate  the  everlasting, 

My  Lords  and  thine. 


ii  FAUST. 

Dost  fondly  fancy  I  shall  hate  my  life, 
And  hie  me  to  the  waste,  because  not  all 
My  blossom-dreams  bear  fruit  ? 

Here  sit  I  forming  manhood  in  ray  image, 

A  race  resembling  me, 

To  sorrow,  and  to  weep, 

To  taste,  to  hold,  to  enjoy, 

And  not  take  heed  of  thoe, 

^a  i  |  GOETHE. 


FAUST. 

Earth's  narrow  circle  is  well  known  to  mo ; 
What  is  above  the  eye  can  never  see. 
Fool,  who  peers  thither  with  his  vision  dim, 
And  feigns  a  crowd  of  beings  like  to  him ! 

Let  him  look  round  him,  standing  without  fear ; 
This  world  speaks  plain  for  who  has  ears  to  hear : 
He  need  not  stray  within  the  vast  to  be, 
But  clasp  what  he  can  feel  and  see. 

So  let  him  wander  all  his  earthly  day ; 

Though  ghosts  should  walk,  still  let  him  go  his  way: 

In  every  progress  woe  and  joy  betide, 

Though  eveiy  moment  be  unsatisfied. 

Y"es,  in  this  thought,  I  fix  unswerving ; 

Wisdom  gives  thus  her  judgment  form  ; 
Those  are  of  Freedom,  Life,  deserving, 

Who  daily  take  them  both  by  storm. 

Goran 


THE  EVOLUTION  OF  MAN. 


CHAPTER   I. 

THE   FUNDAMENTAL   LAW   OF   THE   EVOLUTION   OF 
ORGANISMS. 

General  Significance  of  the  History  of  the  Evolution  of  Man. — Ignorance  of 
it  among  the  so-called  Educated  Classes. — The  Two  Branches  of  the 
History  of  Evolution. — Ontogeny,  or  the  History  of  Germs  (Embryos) , 
and  Phylogeny,  or  the  History  of  Descent  (or  of  the  Tribes). — Causal 
Connection  between  the  Two  Series  of  Evolution. — The  Evolution  of 
the  Tribe  determines  the  Evolution  of  the  Germ. — Ontogeny  as  an 
Epitome  or  Recapitulation  of  Phylogeny.  The  Incompleteness  of  this 
Epitome. — The  Fundamental  Law  of  Biogeny. — Heredity  and  Adapta- 
tion are  the  two  Formative  Functions,  or  the  two  Mechanical  Causes, 
of  Evolution. — Absence  of  Purposive  Causes. — Validity  of  Mechanical 
Causes  only. — Substitution  of  the  Monistic  or  Unitary  for  the  Dualistic, 
or  Binary  Cosmology. — Eadical  Importance  of  the  Facts  of  Embryology 
to  Monistic  Philosophy. — Palingenesis,  or  Derived  History,  and  Keno- 
genesis,  or  Vitiated  History. — History  of  the  Evolution  of  Forms  and 
Functions. — Necessary  Connection  between  Physiogeny  and  Morpho- 
geny. — The  History  of  Evolution  as  yet  almost  entirely  the  Product  of 
Morphology,  and  not  of  Physiology. — The  History  of  the  Evolution  of 
the  Central  Nervous  System  (Brain  and  Spinal  Marrow)  is  involved 
in  that  of  the  Psychic  Activities,  or  the  Mind. 

"  The  History  of  the  Evolution  of  Organisms  consists  of  two  kindred  and 
closely  connected  parts  :  Ontogeny,  which  is  the  history  of  the  evolution  of 
individual  organisms,  and  Phylogeny,  which  is  the  historyof  the  evolution 
of  organic  tribes.  Ontogeny  is  a  brief  and  rapid  recapitulation  of 
Phylogeny,  dependent  on  the  physiological  functions  of  Heredity  (reproduo- 


2  THE   EVOLUTION  OF  MAN. 

tion)  and  Adaptation  (nutrition).  The  individual  organism  reproduces  fn 
the  rapid  and  short  course  of  its  own  evolution  the  most  important  of  the 
changes  in  form  through  which  its  ancestors,  according  to  laws  of  Heredity 
and  Adaptation,  have  passed  in  the  slow  and  long  course  of  their  palaeonto- 
logioal  evolution." — HAECKEL'S  Qenerelle  Morphologic  (1866). 

THE  natural  phenomena  of  the  evolutionary  history  of  man 
claim  an  entirely  peculiar  place  in  the  wide  range  of 
the  scientific  study  of  nature.  There  is  surely  no  subject 
of  scientific  investigation  touching  man  more  closely,  or  in 
the  knowledge  of  which  he  is  more  deeply  concerned,  than 
the  human  organism  itself;  and  of  all  the  various  branches 
of  the  science  of  man,  or  anthropology,  the  history  of 
his  natural  evolution  should  excite  his  highest  interest. 
For  it  affords  a  key  for  the  solution  of  the  greatest  of  those 
problems  at  which  human  science  is  striving.  The  greatest 
problems  with  which  human  science  is  occupied — the  inquiry 
into  the  true  nature  of  man,  or,  as  it  is  called,  the  question 
of  "  Man's  Place  in  Nature,"  which  deals  with  the  past 
and  primitive  history,  the  present  condition,  and  future 
of  Man — are  all  most  directly  and  intimately  linked  to  this 
branch  of  scientific  research,  which  is  called  The  History 
of  the  Evolution  of.  Man,  or  briefly,  "Anthropogeny."1 
It  is,  however  a  most  astonishing  but  incontestable  fact, 
that  the  history  of  the  evolution  of  man  as  yet  constitutes 
no  part  of  general  education.  Indeed,  our  so-called  "  edu- 
cated classes"  are  to  this  day  in  total  ignorance  of  the 
most  important  circumstances  and  the  most  remarkable 
phenomena  which  Anthropogeny  has  brought  to  light. 

In  corroboration  of  this  most  astounding  fact,  I  will 
only  mention  that  most  "educated  people"  do  not  even 
know  that  each  human  individual  is  developed  from  an 
egg,  and  that  this  egg  is  a  simple  cell,  like  that  of  any 


GENERAL  IGNORANCE  OF  THE  HISTORY  OF  EVOLUTION.        3 

animal  or  plant.  They  are  also  ignorant  of  the  fact  that, 
in  the  development  of  this  egg,  an  organism  is  first  formed 
which  is  entirely  different  from  the  fully  developed  human 
body,  to  which  it  bears  no  trace  of  resemblance.  The 
majority  of  "educated  people"  have  never  seen  such  a 
human  germ,  or  embryo,  in  the  early  stages  of  development,2 
nor  are  they  aware  that  it  is  not  at  all  different  from  those 
of  other  animals.  They  do  not  know  that,  at  a  certain 
period,  this  embryo  has  essentially  the  anatomical  structure 
of  a  Lancelot,  later  of  a  Fish,  and  in  subsequent  stages 
those  of  Amphibian  and  Mammal  forms ;  and  that  in  the 
further  evolution  of  these  mammal  forms  those  first  appear 
which  stand  lowest  in  the  series,  namely,  forms  allied  to 
the  Beaked  Animals  (Ornithorhynchus) ;  then  those  allied 
to  Pouched  Animals  (Marsupialia),  which  are  followed  by 
forms  most  resembling  Apes ;  till  at  last  the  peculiar  human 
form  is  produced  as  the  final  result.  These  significant  facts 
are  so  little  known  that,  when  incidentally  mentioned,  they 
are  commonly  doubted,  or  are  even  regarded  as  unfounded 
inventions.  Every  one  knows  that  the  butterfly  proceeds 
from  a  pupa,  the  pupa  from  a  caterpillar,  to  which  it  bears 
no  resemblance,  and  again  the  caterpillar  from  the  egg  of  the 
butterfly.  But  few,  except  those  of  the  medical  profession, 
are  aware  that  man,  in  the  course  of  his  individual  evolution, 
passes  through  a  series  of  transformations  no  less  astonishing 
and  remarkable  than  the  well-known  metamorphoses  of  the 
butterfly.  The  mere  tracing  of  this  wonderful  series  of  forms, 
through  which  the  human  embryo  passes  in  the  course  of  its 
development,  is,  of  course,  of  great  general  interest.  But  our 
understanding  will  be  satisfied  in  a  far  higher  degree,  if  we 
refer  these  remarkable  facts  to  their  final  causes,  and  recognize 


4  THE  EVOLUTION  OF  MAN. 

that  these  natural  phenomena  are  of  the  utmost  importance 
to  the  entire  range  of  human  knowledge.  They  are  of 
special  importance  to  the  "  History  of  Creation,"  and,  in 
connection  with  this,  to  philosophy  in  general, — as  we  shal] 
presently  see.  Further,  as  the  general  results  of  all  human 
striving  after  knowledge  are  summed  up  in  philosophy,  it 
follows  that  every  branch  of  scientific  research  comes  more 
or  less  in  contact  with,  and  is  influenced  by,  the  History  of 
the  Evolution  of  Man. 

In  undertaking  to  describe  the  most  important  character- 
istics of  these  significant  phenomena,  and  to  trace  them 
back  to  their  final  causes,  I  shall  assign  a  much  greater 
scope  and  aim  to  the  History  of  the  Evolution  of  Man  than 
is  usual.  The  lectures  given  on  this  subject  in  German 
universities  during  the  past  fifty  years  have  been  exclusively 
designed  for  medical  students.  It  is  true  that  the  physician 
is  most  deeply  interested  in  becoming  acquainted  with  the 
development  of  the  bodily  organization  of  man,  with  which 
he  deals,  practically,  from  day  to  day,  in  his  profession.  I 
shall  not  here  attempt  to  give  a  special  account  of  the  course 
of  the  evolution  of  the  individual,  such  as  has  usually  been 
given  in  embryological  lectures,  because  few  of  my  readers 
have  studied  human  anatomy,  or  are  acquainted  with 
the  physical  structure  of  the  developed  man.  Hence,  I 
shall  have  to  confine  myself  in  many  points  to  general 
outlines,  neglecting  many  of  the  remarkable  details,  which 
would  have  to  be  discussed  in  treating  of  the  evolution  of 
special  human  organs,  but  which  from  their  complicated 
nature,  and  because  they  are  not  easy  to  describe,  can  only 
be  completely  understood  by  the  aid  of  an  intimate  ac- 
quaintance with  human  anatomy.  I  shall  strive,  however 


OBJECT   OF   THE   HISTORY   OF   EVOLUTION.  5 

to  present  this  branch  of  the  science  in  as  popular  a  form  as 
possible.  A  satisfactory  general  idea  of  the  course  of  the 
evolution  of  the  human  embryo  can,  indeed,  be  given  without 
going  very  deeply  into  anatomical  details.  As  numerous 
successful  attempts  have  recently  been  made  to  awaken 
the  interest  of  larger  classes  of  educated  persons  in  other 
branches  of  Science,  I  also  may  hope  to  succeed  in  this 
department,  though  it  is  in  many  respects  especially  beset 
with  difficulties. 

The  History  of  the  Evolution  of  Man,  as  it  has  been 
usually  treated  in  lectures  for  medical  students  at  the 
universities,  has  only  concerned  itself  with  Embryology,3 
so-called,  or  more  correctly  with  Ontogeny,4  in  other  words, 
with  the  history  of  the  evolution  of  individual  human 
organisms.  This,  however,  is  only  the  first  part  of  the  task 
before  us,  only  the  first  half  of  the  History  of  the  Evolution 
of  Man  in  the  wider  sense  which  will  here  be  attributed 
.0  the  term.  The  second  part,  equal  in  importance  and 
interest,  is  Phylogeny,5  which  is  the  history  of  the  evolution 
of  the  descent  of  man,  that  is,  of  the  evolution  of  the 
various  animal  forms  through  which,  in  the  course  of  count- 
less ages,  mankind  has  gradually  passed  into  its  present 
form.  All  my  readers  know  of  the  very  important  scientific 
movement  which  Charles  Darwin  caused  fifteen  years  ago, 
by  his  book  on  the  Origin  of  Species.  The  most  important 
direct  consequence  of  this  work,  which  marks  a  fresh  epoch, 
has  been  to  cause  new  inquiries  to  be  made  into  the 
origin  of  the  human  race,  which  have  proved  the  natural 
evolution  of  man  through  lower  animal  forms.  The  Science 
which  treats  of  the  development  of  the  human  race  from 
the  animal  kingdom  is  called  Phylogeny,  or  the  tribal 


6  THE  EVOLUTION   OF  MAN. 

history  of  man.  The  most  important  source  from  which 
the  science  derives  its  material,  is  Ontogeny,  or  the  history 
of  germs,  in  other  words,  of  the  evolution  of  the  individual. 
Palaeontology,  or  the  science  of  petrifactions,  and,  in  a  yet 
greater  degree,  Comparative  Anatomy,  also  afiord  most  im- 
portant aid  to  Phylogeny. 

These  two  divisions  of  our  science,  Ontogeny,  or  the 
history  of  the  germ,  Phylogeny,  or  the  history  of  the 
tribe,  are  most  intimately  connected,  and  the  one  cannot 
be  understood  without  the  other.  The  close  intertwining 
of  both  branches,  the  increased  proportions  which  germ- 
history  and  tribal  history  lend  to  each  other,  alone  raise 
Biogeny  6  (or  the  history  of  organic  evolution,  in  the  widest 
sense)  to  the  rank  of  a  philosophic  natural  science.  The 
connection  between  the  two  is  not  external  and  superficial, 
but  deeply  internal  and  causal  Our  knowledge  of  this 
connection  has  been  but  very  recently  obtained ;  it  is  most 
clearly  and  accurately  expressed  in  the  comprehensive  state- 
ment which  I  call  "the  fundamental  law  of  organic 
evolution"  or  more  briefly,  "  the  first  principle  of  Biogeny."  7 

This  fundamental  law,  to  which  we  shall  recur  again 
and  again,  and  on  the  recognition  of  which  depends  the 
thorough  understanding  of  the  history  of  evolution,  is  briefly 
expressed  in  the  proposition  :  that  the  History  of  the  Gerin 
is  an  epitome  of  the  History  of  the  Descent ;  or,  in  other 
words :  that  Ontogeny  is  a  recapitulation  of  Phylogeny ;  or, 
somewhat  more  explicitly :  that  the  series  of  forms  through 
which  the  Individual  Organism  passes  during  its  progress  from 
the  egg  cell  to  its  fully  developed  state,  is  a  brief,  compressed 
reproduction  of  the  long  series  of  forms  through  which  the 
animal  ancestors  of  that  organism  (or  the  ancestral  forms 


THE  INFLUENCE  OF  PHYLOGENY  ON  ONTOGENY.     7 

of  its  species)  have  passed  from  the  earliest  periods  of  so- 
called  organic  creation  down  to  the  present  time. 

The  causal  nature  of  the  relation  which  connects  the 
History  of  the  Germ  (Embryology,  or  Ontogeny)  with  that 
of  the  tribe  (Phylogeny)  is  dependent  on  the  phenomena 
of  Heredity  and  Adaptation.  When  these  are  properly 
understood,  and  their  fundamental  importance  in  deter- 
mining the  forms  of  organisms  recognized,  we  may  go 
a  step  further,  and  say:  Phylogenesis  is  the  mechanical 
cause  of  Ontogenesis.  The  Evolution  of  the  Tribe,  which 
is  dependent  on  the  laws  of  Heredity  and  Adaptation,  effects 
all  the  events  which  take  place  in  the  course  of  the  Evolution 
of  the  Germ  or  Embryo. 

The  chain  of  different  animal  forms  which,  according  to 
the  Theory  of  Descent,  constitutes  the  series  of  ancestors,  or 
chain  of  forefathers  of  every  higher  organism,  and  hence 
also  of  man,  always  forms  a  connected  whole.  This  un- 
broken succession  of  forms  may  be  represented  by  the  letters 
of  the  Alphabet  A,  B,  C,  D,  E,  etc.,  down  to  Z,  in  their 
alphabetical  order.  In  apparent  contradiction  to  this,  the 
history  of  the  individual  evolution,  or  the  Ontogeny  of  most 
organisms  show  us  only  a  fragment  of  this  series  of  forms,  so 
that  the  interrupted  chain  of  embryonic  forms  would  be 
represented  by  something  like :  A,  B,  F,  H,  I,  K,  L,  etc. ;  or, 
in  other  cases,  thus :  B,  D,  H,  L,  M,  N,  etc.  Several  evolu- 
tionary forms  have,  therefore,  usually  dropped  out  of  t'he 
originally  unbroken  chain  of  forms.  In  many  cases  also 
(retaining  the  figure  of  the  repeated  alphabet)  one  or  more 
letters,  representing  ancestral  forms,  are  replaced  in  the 
corresponding  places  among  the  embryonic  forms  by  equi- 
valent letters  of  another  alphabet.  Thus,  for  example,  in 


8  THE  EVOLUTION   OF  MAN. 

place  of  the  Latin  B  or  D,  a  Greek  B  or  A  is  often  found 
Here,  therefore,  the  text  of  the  biogenetic  first  principle  is 
vitiated,  while  in  the  former  case  it  was  epitomized  This 
gives  more  importance  to  the  fact  that,  notwithstanding 
this,  the  sequence  remains  the  same,  so  that  we  are  enabled 
to  recognize  its  original  order. 

Indeed,  there  is  always  a  complete  parallelism  between  the 
t  wo  series  of  evolution.  This  is,  however,  vitiated  by  the 
fact  that  in  most  cases  many  forms  which  formerly  existed 
and  actually  lived  in  the  phylogenetic  series  are  now  wanting, 
and  have  been  lost  from  the  ontogenetic  series  of  evolution. 
If  the  parallelism  between  the  two  series  were  perfect,  and 
if  this  great  fundamental  law  of  the  causal  connection  between 
Ontogeny  and  Phylogeny,  in  the  strict  sense  of  the  word, 
had  full  and  unconditional  sway,  we  should  only  have  to 
ascertain,  with  the  aid  of  microscope  and  scalpel,  the  series  of 
forms  through  which  the  fertilized  human  egg  passes  before 
it  attains  its  complete  development.  Such  an  examination 
would  at  once  give  us  a  complete  picture  of  the  remarkable 
series  of  forms  through  which  the  animal  ancestors  of  the 
human  race  have  passed,  from  the  beginning  of  organic 
creation  to  the  first  appearance  of  man.  But  this  repro- 
duction of  the  Phylogeny  in  the  Ontogeny  is  complete  only 
in  rare  instances,  and  seldom  corresponds  to  the  entire  series 
of  the  letters  of  the  alphabet.  In  fact,  in  most  cases  the 
epitome  is  very  incomplete,  and  greatly  altered  and  per- 
verted by  causes  which  we  shall  investigate  hereafter.  Hence 
we  are  seldom  able  to  determine  directly,  by  means  of  its 
Ontogeny,  the  different  forms  through  which  the  ancestry  of 
each  organism  has  passed;  on  the  contrary,  we  commonly 
find, — and  not  less  so  in  the  Phylogeny  of  man, — a  number 


CONNECTION  BETWEEN  PHYLOGENY  AND  ONTOGENY.     9 

of  gaps.  We  are,  however,  able  to  bridge  over  the  greater 
part  of  these  gaps  satisfactorily  by  the  help  of  Compa- 
rative Anatomy,  though  not  to  fill  them  up  directly  by 
ontogenetic  research.  It  is  therefore  all  the  more  im- 
portant that  we  are  acquainted  with  a  considerable  number 
of  lower  animal  forms  which  still  find  place  in  the  history  of 
the  individual  evolution  of  man.  In  such  cases,  from  the 
nature  of  the  transient  individual  form,  we  may  quite  safely 
infer  the  nature  of  the  ancestral  animal  form. 

For  example,  from  the  fact  that  the  human  egg  is  a 
simple  cell,  we  may  at  once  infer  that  there  has  been  at  a 
very  remote  time  a  unicellular  ancestor  of  the  human  race 
resembling  an  Amoeba.  Again,  from  the  fact  that  the 
human  embryo  originally  consists  merely  of  two  simple 
germ-layers,  we  may  at  once  safely  infer  that  a  very  ancient 
ancestral  form  is  represented  by  the  two-layered  Gastrsea.  A 
later  embryonic  form  of  the  human  being  points  with  equal 
certainty  to  a  primitive  worm-like  ancestral  form  which  is 
related  to  the  sea-squirts  or  Ascidians  of  the  present  day. 
But  the  low  animal  forms  which  constitute  the  ancestral 
line  between  the  unicellular  amceba  and  the  gastrsea,  and 
further  between  the  gastrsea  and  the  ascidian  form,  can  only 
be  approximately  conjectured  with  the  aid  of  Comparative 
Anatomy  and  Ontogeny.  On  account  of  a  shortened  process 
of  Heredity,  various  ontogenetic  intermediate  forms,  which 
must  have  existed  phylogenetically,  or  in  the  ancestral 
lineage,  have  in  the  course  of  historic  evolution  gradually 
dropped  out  from  these  gaps.  But  notwithstanding  these 
numerous  and  sometimes  very  considerable  gaps,  there  is,  on 
the  whole,  complete  agreement  between  the  two  series  of 
evolution.  Indeed,  it  will  be  one  of  my  principal  objects  to 


IO  THE   EVOLUTION   OF   MAN. 

prove  the  deep  harmony,  and  original  parallelism,  be- 
tween the  two  series.  By  adducing  numerous  facts,  I  hope 
to  convince  my  readers  that  from  the  actually  existing 
series  of  embryonic  forms  which  can  be  shown  at  any  time, 
we  are  able  to  draw  the  most  important  conclusions  as  to 
the  genealogical  tree  of  the  human  species.  We  shall  thus 
be  able  to  form  a  general  picture  of  the  series  of  animal 
forms  which  succeeded  each  other  as  the  direct  ancestors  of 
man,  in  the  long  course  of  the  history  of  the  organic  world. 

In  this  phylogenetic  significance  of  ontogenetic  phe- 
nomena, it  is  of  course  most  important  to  distinguish  clearly 
and  exactly  between  the  original,  palingenetic  processes  of 
evolution,  and  the  later  kenogenetic  processes  of  the  same. 
The  term  Palingenetic  process8  (or  reproduction  of  the  history 
of  the  germ)  is  applied  to  all  such  phenomena  in  the  history 
of  evolution  as  are  exactly  reproduced,  in  consequence  of 
conservative  heredity,  in  each  succeeding  generation,  and 
which,  therefore,  enables  us  directly  to  infer  the  corre- 
sponding processes  in  the  tribal  history  of  the  developed 
ancestors.  The  term  Kenogenetic  process9  (or  vitiation  of 
the  history  of  the  germ)  is  applied  to  all  such  processes  in 
the  germ-history  as  are  not  to  be  explained  by  heredity 
from  primaeval  parent-forms,  but  which  have  been  acquired 
at  a  later  time  in  consequence  of  the  adaptation  of  the 
germ,  or  embryo  form,  to  special  conditions  of  evolution 
These  kenogenetic  processes  are  recent  additions,  which  do 
not  allow  of  direct  inference  as  to  the  corresponding  pro- 
cesses in  the  tribal  history  of  the  ancestral  line,  but  which 
i-ather  falsify  and  conceal  the  latter. 

This  critical  distinction  between  the  primary  palinge- 
netic, and  the  secondary  kenogenetic  processes  is  of  course 


PALINGENESIS  AND   KENOGEXESIS.  II 

of  the  greatest  importance  to  scientific  Phylogeny,  which, 
from  the  available  empiric  material  supplied  by  Ontogeny, 
by  Comparative  Anatomy,  and  by  Palaeontology,  seeks  to 
infer  the  long  extinct  historical  processes  of  tribal  evolution. 
It  is  of  the  same  importance  to  the  student  of  evolution 
as  is  the  critical  distinction  between  corrupt  and  genuine 
passages  in  the  text  of  an  old  writer  to  the  philologist ;  the 
separation  of  the  original  text  from  interpolations  and  corrupt 
readings.  This  distinction  between  Palingenesis  or  inherited 
evolution,  and  Kenogenesis  or  vitiated  evolution,  has  not, 
however,  yet  been  sufficiently  appreciated  by  naturalists. 
But  I  believe  that  it  is  the  first  condition  requisite,  if  the 
history  of  evolution  is  to  be  really  understood,  and  I  think 
that  two  separate  main  divisions,  based  on  this  distinction, 
must  be  made  in  germ-history;  Palingenesis  or  inherited 
history,  and  Kenogenesis  or  vitiated  history. 

Let  us  illustrate  this  highly  important  distinction  by  a 
few  examples  taken  from  the  evolution  of  man.  In  Man,  as  in 
all  other  higher  Vertebrates,  the  following  incidents  of  germ- 
history  must  be  regarded  as  palingenetic  processes :  the 
formation  of  the  two  primary  germ-layers,  the  appearance 
of  a  simple  notochord  (Chorda)  between  the  spinal  tube  and 
the  intestinal  tube,  the  transitory  formation  of  gill-arches 
and  gill-openings,  of  primitive  kidneys,  of  the  primitive  brain 
bladder,  the  hermaphrodite  rudiment  of  the  sexual  organSj 
etc  All  these,  and  many  other  important  phenomena  have 
evidently  been  accurately  handed  down,  by  constant  heredity, 
from  the  primaaval  ancestors  of  Mammals,  and  must,  there- 
fore, be  referred  directly  to  corresponding  palseontological 
evolutionary  incidents  in  the  history  of  the  tribe.  On  the 
other  hand,  this  is  not  the  case  with  the  following  germinal 


12  THE   EVOLUTION   OF   MAN. 

incidents,  which  must  be  explained  as  kenogenetie  pro- 
cesses ;  the  formation  of  the  yelk- sac,  of  the  allantois  and 
placenta,  of  the  amnion  and  chorion,  and,  generally,  of  the 
different  egg-membranes  and  the  corresponding  systems  of 
blood-vessels;  also  the  transitory  separation  of  the  primitive 
vertebrate  plates  and  the  side-plates,  the  secondary  closing 
of  the  stomach  wall  and  the  intestinal  wall,  the  formation 
of  the  navel,  etc.  All  these,  and  many  other  phenomena 
are  evidently  not  referable  to  corresponding  conditions  of 
an  earlier,  independent,  and  fully  developed  parent  form, 
but  must  be  explained  as  solely  due  to  adaptation  to  the 
peculiar  conditions  of  egg-life  or  embryo-life  (within  the 
egg-membranes).  With  reference  to  this  fact  we  may  now 
define  our  "first  principle  of  Biogeny"  more  exactly  as 
follows :  "  The  evolution  of  the  germ  (Ontogeny)  is  a  com- 
pressed and  shortened  reproduction  of  the  evolution  of  the 
tribe  (Phylogeny) ;  and,  moreover,  this  reproduction  is  more 
complete,  in  proportion  as,  in  consequence  of  constant 
heredity,  the  original  inherited  evolution  (Palingenesis)  is 
more  closely  retained;  on  the  other  hand,  the  repetition 
is  more  incomplete,  in  proportion  as  the  later  vitiated 
evolution  (Kenogenesis)  is  introduced  by  changing  adapta- 
tion."10 

The  kenogenetic  vitiations  of  the  original,  palingenetic 
incidents  of  evolution  depend  in  great  measure  on  a  gradually 
occurring  displacement  of  the  phenomena,  which  is  effected 
in  the  course  of  many  thousands  of  years  by  adaption  to  the 
changed  conditions  of  embryonic  existence.  This  displace- 
ment may  effect  either  the  place  or  the  time  of  the 
phenomena.  If  the  former,  it  is  called  Heterotopy ;  if  the 
latter,  Heterochrony. 


HETEROTOPY  AND  HETEROCHRONY.          13 

"  Displacement  in  position,  "  or  "  Heterotopy,"  especially 
affects  the  cells  or  elementary  parts  which  compose  the 
organs;  but  it  also  affects  the  organs  themselves.  For 
example,  the  sexual  organs  of  the  human  embryo,  as  well  as 
those  of  many  higher  animals,  appear  to  originate  from 
the  middle  germ-layer.  But  the  comparative  Ontogeny  of 
the  lower  animals  shows,  on  the  other  hand,  that  these 
organs  did  not  originally  arise  from  this  layer,  but  from  one 
of  the  primary  germ-layers ;  the  male  sexual  organs  from 
the  outer  germ-layer,  the  female  from  the  inner.  Gradually, 
however,  the  germ-cells  have  altered  their  original  site,  and 
have  made  their  way,  at  an  early  period,  from  their  original 
position  into  the  middle  germ-layer,  so  that  they  now 
appear  actually  to  originate  in  the  latter.  An  analogous 
heterotopism  affects  the  primitive  kidneys  in  the  higher 
Vertebrates.  Even  the  appearance  of  the  mesoderm  itself 
is  very  greatly  affected  by  a  displacement  in  position,  which 
is  connected  with  the  transition  of  embryo  cells  from  one 
germ-layer  into  another. 

The  kenogenetic  "  displacements  in  time,"  or  "  Hetero- 
chronisms,"  are  equally  significant.  They  are  seen  in  the 
fact  that  in  the  germ-history  (Ontogeny)  the  sequence  in 
which  the  organs  appears  differs  from  that  which,  judging 
from  the  tribal  history  (Phylogeny),  would  be  expected.  By 
hcterotopy  the  sequence  in  position  is  vitiated;  similarly, 
by  heterochrony  the  sequence  in  time  is  vitiated.  This 
vitiation  may  effect  either  an  acceleration  or  a  retardation 
in  the  appearance  of  the  organs.  We  must  regard  the 
following  incidents  in  the  germ-history  of  man  as  examples 
of  ontogenetic  acceleration :  the  early  appearance  of  the 
heart,  the  gill-openings,  the  brain,  the  eyes,  the  chorda, 


14  THE  EVOLUTION   OF   MAN. 

etc.  It  is  evident  that  these  organs  appear  earlier  in 
relation  to  others  than  was  originally  the  case  in  the 
history  of  the  tribe.  The  reverse  is  true  of  the  retarded 
completion  of  the  intestinal  canal,  the  body-cavity,  and  the 
sexual  organs.  It  is  evident  that  in  these  cases  there  is  an 
ontogeuetic  postponement  or  retardation. 

It  is  only  by  critically  appreciating  these  kenogenetic 
incidents  in  relation  to  the  palingenetic,  and  by  constantly 
allowing  for  the  changes  in  inherited  evolution  effected 
by  vitiated  evolution,  that  it  is  possible  to  recognize  the 
fundamental  significance  of  the  first  principle  of  Biogeny, 
which  in  this  way  attains  its  true  value  as  the  most  im- 
portant explanatory  principle  of  the  history  of  evolution. 
When  it  is  thus  critically  appreciated,  this  first  principle 
also  proves  to  be  the  "  red  thread  "  on  which  we  can  string 
every  one  of  the  phenomena  in  this  wonderful  domain ; 
this  is  the  thread  of  Ariadne,  with  the  aid  of  which  alone 
we  are  able  to  find  an  intelligible  course  through  this  com- 
plicated labyrinth  of  forms.  Even  at  an  earlier  period,  when 
the  history  of  the  evolution  of  the  human  and  the  animal 
individual  first  became  somewhat  more  accurately  known — 
which  is  hardly  half  a  century  ago ! — people  were  greatly 
surprised  at  the  wonderful  similarity  existing  in  the  onto- 
genetic  forms,  or  the  stages  of  the  individual  evolution,  of 
very  different  animals.  They  noticed  also  the  remarkable 
resemblance  between  these  and  certain  developed  animal 
forms  of  allied  lower  groups.  Even  the  older  natural  philo- 
sophers recognized  the  fact  that  in  a  certain  way  these 
lower  animals  permanently  represent  in  the  system  of  the 
animal  kingdom  forms  which  appear  transiently  in  the 
evolution  of  individuals  of  higher  groups.  But  formerly 


HEREDITY   AND   ADAPTATION.  15 

it  was  impossible  to  understand  and  interpret  aright  this 
remarkable  resemblance.  Darwin's  greatest  merit  is  that 
he  has  now  enabled  us  to  understand  this  circumstance. 
This  gifted  naturalist  was  the  first  to  place  the  pheno- 
mena of  Heredity  on  the  one  hand,  and  of  Adaptation  on 
the  other,  in  their  true  light,  and  to  show  the  fundamental 
significance  of  their  constant  interaction  in  the  production 
of  organic  forms.  He  was  the  first  to  point  out  the  im- 
portant part  played  by  the  continual  Struggle  for  Existence 
in  which  all  organisms  take  part,  and  how  under  its  in- 
fluence, through  Natural  Selection,  new  species  of  organisms 
have  arisen,  and  still  arise,  entirely  by  the  interaction  of 
Heredity  and  Adaptation.  Darwin  thus  enabled  us  properly 
to  understand  the  immensely  important  relation  existing 
between  the  two  divisions  of  the  History  of  Evolution : 
Ontogeny,  and  Phylogeny. 

If  the  phenomena  of  Heredity  and  Adaptation  are  left 
unnoticed,  if  these  two  formative  physiological  functions  of 
the  organism  are  not  taken  into  account,  then  it  is  entirely 
impossible  thoroughly  to  understand  the  History  of  Evolution; 
so  that  before  the  time  of  Darwin  we  had  no  clear  idea  of 
the  real  nature  and  caus-es  of  the  development  of  germs. 
It  was  utterly  impossible  to  explain  the  strange  series  of 
forms  through  which  a  human  being  passes  in  its  embryonic 
evolution ;  it  was  impossible  to  comprehend  the  reason  of 
the  curious  series  of  various  animal-like  forms  which  appear 
in  the  Ontogeny  of  man.  Previously  it  was  even  generally 
believed  that  the  whole  human  being,  with  all  its  parts 
foreshadowed,  existed  even  in  the  egg,  and  that  his  evolution 
was  only  an  unfolding  of  the  form,  a  simple  process  of 
growth.  But  this  is  not  at  all  the  case.  On  the  contrary, 


1 6  THE  EVOLUTION   OF  MAN. 

the  entire  process  of  the  evolution  of  the  individual  presents 
to  the  eye  a  connected  series  of  diverse  animal  forms  ;  and 
these  various  animal  forms  exhibit  very  diverse  conditions 
of  external  and  internal  structure.  The  reason  why  every 
human  individual  must  pass  through  this  series  of  forms  in 
the  course  of  his  embryonic  evolution,  was  first  explained 
to  us  by  the  Theory  of  Descent  of  Lamarck  and  Darwin. 
From  this  theory  we  first  learn  the  efficient  causes  (causce 
effiaientes)  of  individual  evolution ;  by  the  aid  of  this  theory 
we  first  perceive  that  such  mechanical  causes  alone  suffice 
to  effect  the  evolution  of  the  individual  organism,  and 
that  the  co-operation  of  designing,  or  teleological  causes 
(causce  finales),  which  were  formerly  universally  assumed, 
is  unnecessary.  Of  course,  these  final  causes  still  play  an 
important  part  in  the  prevailing  school-philosophy ;  but  in 
our  new  natural  philosophy  we  are  enabled  to  replace  them 
entirely  by  the  efficient  causes. 

I  allude  to  this  matter  at  this  early  stage,  in  order  to 
call  attention  to  one  of  the  most  important  advances  made  in 
any  branch  of  human  knowledge  during  the  past  ten  years. 
The  history  of  philosophy  shows  that  in  the  cosmology  of 
our  day,  as  in  that  of  antiquity,  final  causes  are  almost 
universally  deemed  to  be  the  real  ultimate  causes  of  the 
phenomena  of  organic  life,  and  especially  those  of  the  life 
of  man.  The  prevailing  Doctrine  of  Design,  or  Teleology, 
assumes  that  the  phenomena  of  organic  life,  and  in  particular 
those  of  evolution,  are  explicable  only  by  purposive  causes, 
and  that,  on  the  contrary,  they  in  no  way  admit  of  a 
mechanical  explanation,  that  is,  one  entirely  based  on 
natural  science.  The  most  difficult  problems  in  this  respect 
which  have  been  before  us,  and  which  seemed  capable  of 


MONISM   AND   DUALISM.  \J 

solution  only  by  means  of  Teleology,  are,  however,  precisely 
those  which  have  been  mechanically  solved  in  the  Theory 
of  Descent.  The  reconstruction  of  the  history  of  the  evolu- 
tion of  man,  which  this  theory  has  effected,  has  actually 
removed  the  greatest  difficulties.  We  shall  see  in  the 
course  of  our  inquiries  how,  through  Darwin's  reform  of 
the  Doctrine  of  Evolution,  the  most  wonderful  problems, 
hitherto  deemed  unapproachable,  of  the  organization  of 
man  and  animals  have  admitted  of  a  natural  solution,  of  a 
mechanical  explanation,  by  non-purposive  causes.  It  has 
enabled  us  to  substitute  everywhere  unconscious  causes 
acting  from  necessity,  for  conscious  purposive  causes.11 

If  the  recent  progress  in  the  Doctrine  of  Evolution  had 
accomplished  only  this,  every  thoughtful  person  must  have 
admitted  that  even  in  this  an  immense  advance  had  been 
made  in  knowledge.  In  consequence  of  it,  the  tendency 
called  unitary  or  monistic,  in  contradistinction  to  the  dual- 
istic,  or  binary,  which  has  heretofore  prevailed  in  speculative 
philosophy,  must  ultimately  prevail  throughout  philosophy.12 
This  is  the  point  at  which  the  history  of  the  evolution  of 
man  at  once  penetrates  deeply  into  the  very  foundations 
of  philosophy.  For  this  reason  alone  it  is  very  much  to  be 
desired,  in  fact  is  indispensable,  that  any  one  who  aspires  to 
philosophic  culture  should  learn  the  most  important  facts  in 
this  field  of  research. 

The  significance  of  the  facts  of  Ontogeny  is  so  great  and 
so  evident  that  the  dualistic  teleological  philosophy,  finding 
them  extremely  inconvenient,  has  of  late  endeavoured  to 
meet  them  by  simple  denial  Such,  for  instance,  has  been 
the  case  with  the  fact  that  every  human  being  develops 
from  an  egg,  and  that  this  egg  is  a  simple  cell,  like  the  3gg- 


1 8  THE   EVOLUTION   OF   MAN. 

cell  of  all  other  animals.  When  in  my  "History  of  Creation" 
I  had  discussed  this  fundamental  fact,  and  had  directed 
attention  to  its  immense  significance,  several  theological 
periodicals  pronounced  it  a  malicious  invention  of  my  own, 
The  evident  fact  that  at  a  certain  stage  of  their  evolution 
the  embryos  of  Man  and  of  the  Dog  are  entirely  in- 
distinguishable from  one  another  was  also  denied. 

The  fact  is  that  an  examination  of  the  human  embryo  in 
the  third  or  fourth  week  of  its  evolution  shows  it  to  be 
altogether  different  from  the  fully  developed  Man,  and  that 
it  exactly  corresponds  to  the  undeveloped  embryo-form 
presented  by  the  Ape,  the  Dog,  the  Rabbit,  and  other 
Mammals,  at  the  same  stage  of  their  Ontogeny.  At  this 
stage  it  is  a  bean-shaped  body  of  very  simple  structure, 
with  a  tail  behind,  and  two  pairs  of  paddles,  resembling  the 
fins  of  a  fish,  and  totally  dissimilar  to  the  limbs  of  man  and 
other  mammals,  at  the  sides.  Nearly  the  whole  of  the  front 
half  of  the  body  consists  of  a  shapeless  head  without  a  face, 
on  the  sides  of  which  are  seen  gill-fissures  and  gill-arches 
as  in  Fishes.  (Of.  Plate  VII.  at  the  end  of  Chapter  XI.) 
In  this  stage  of  evolution  the  human  embryo  differs  in  no 
essential  way  from  the  embryo  of  an  Ape,  Dog,  Horse,  Ox, 
etc.,  at  a  corresponding  age.  Even  such  facts  as  these., 
which  can  be  easily  and  promptly  demonstrated  at  any  time 
by  placing  side  by  side  the  corresponding  embryos  of  Man, 
a  Dog,  a  Horse,  etc.,  have  been  spoken  of  by  theologians 
and  teleological  philosophers  as  inventions  of  materialism  ; 
and  even  naturalists,  who  were  presumably  acquainted  with 
them,  have  tried  to  deny  them.  No  stronger  proof,  surely, 
of  the  immense  radical  importance  of  these  embryological 
facts,  in  favour  of  the  monistic  philosophy  can  be  given  than 


INTERRELATION    OF   FORMS   AND   FUNCTIONS.  19 

these  efforts  on  the  part  of  the  dualistic  school  to  meet  them 
by  simple  denial  or  utter  silence.  They  are  indeed 
extremely  distasteful  to  that  school,  and  are  totally 
irreconcilable  with  their  teleological  cosmology.  We  must 
therefore  take  especial  care  to  place  them  in  their  true  light. 
We  are  entirely  of  the  opinion  of  Huxley,  who,  in  his  able 
"  Evidence  as  to  Man's  Place  in  Nature,"  says  that  these 
facts,  "  though  ignored  by  many  of  the  professed  instructors 
of  the  public  mind,  are  easy  of  demonstration,  and  are 
universally  agreed  to  by  men  of  science;  while  their 
significance  is  so  great,  that  whoso  has  deeply  pondered 
over  them  will,  I  think,  find  little  to  startle  him  in  the 
other  revelations  of  Biology." 

Although  our  chief  inquiry  is  primarily  directed  to  the 
history  of  the  evolution  of  the  bodily  form  of  Man  and  of 
his  organs,  and  to  their  external  and  internal  structural 
relations,  I  must  here  at  once  observe  that  the  history  of 
the  evolution  of  the  functions  is  inseparably  connected  with 
this.  Everywhere  in  Anthropology,  just  as  in  Zoology,  of 
which  the  former  is  but  a  part,  and  throughout  the  whole 
field  of  Biology,  these  two  branches  of  research  are  thus 
inseparably  connected.  The  peculiar  form  of  the  organism 
and  its  organs,  both  internal  and  external,  is  always  closely 
related  to  the  peculiar  manifestations  of  life,  of  the  organism 
and  its  organs,  or,  in  other  words,  to  the  physiological  func- 
tions performed  by  these.  This  intimate  relation  between 
form  and  function  is  also  shown  in  the  evolution  of  the  organ- 
ism and  its  various  parts.  The  history  of  the  evolution  ot 
forms,  which  primarily  occupies  us,  is  at  the  same  time  the 
history  of  the  evolution  of  functions ;  and  this  is  equally 
true  of  the  human  and  of  all  other  organisms. 


20  THE  EVOLUTION  OF  MAN. 

But  I  must  here  add  at  once,  that  our  knowledge  of  th« 
evolution  of  functions  is  as  yet  far  from  being  so  advanced 
as  our  knowledge  of  the  evolution  of  forms.  Indeed,  properly 
speaking,  the  entire  history  of  evolution,  or  Biogeny,  includ- 
ing both  Ontogeny  and  Phylogeny,  has  as  yet  been  almost 
exclusively  a  history  of  the  evolution  of  forms,  while  the 
Biogeny  of  functions  hardly  exists  even  in  name.  The  fault 
lies  solely  with  Physiology,  which  has  as  yet  scarcely  given 
a  thought  to  the  history  of  evolution,  which  it  has  left 
entirely  to  the  care  of  Morphology. 

The  two  chief  divisions  of  biological  research — Mor- 
phology and  Physiology — have  long  travelled  apart,  taking 
different  patha  This  is  perfectly  natural,  for  the  aims,  as 
well  as  the  methods,  of  the  two  divisions  are  different. 
Morphology,  the  science  of  forms,  aims  at  a  scientific  under- 
standing of  organic  structures,  of  their  internal  and  external 
proportions  of  form.  Physiology,  the  science  of  functions, 
on  the  other  hand,  aims  at  a  knowledge  of  the  functions 
of  organs,  or,  in  other  words,  of  the  manifestations  of  life.13 
Physiology,  however,  has,  especially  during  the  last  twenty 
years,  been  far  more  one-sided  in  its  progress  than  Mor- 
phology. Not  only  has  it  entirely  neglected  to  apply  the 
comparative  method,  by  which  Morphology  has  gained  its 
greatest  results,  but  it  has  altogether  disregarded  the  History 
of  Evolution.  Hence  it  has  come  to  pass  that,  within  the 
past  few  decades,  Morphology  has  advanced  far  beyond 
Physiology,  although  the  latter  is  pleased  to  look  haughtily 
down  upon  the  former.'  It  is  Morphology  which  has  gained 
the  greatest  results  in  the  fields  of  Comparative  Anatomy 
and  Biogeny,  and  almost  everything  stated  in  these  pages 
as  to  the  History  of  the  Evolution  of  Man.  is  due  to  the 


DEFECTIVE   STATE   OF   PHYSIOLOGY.  21 

exertions  of  morphologists,  and  not  of  physiologists.  Indeed 
the  direction  at  present  taken  by  Physiology  is  so  one- 
sided that  it  has  even  neglected  the  recognition  of  the  most 
important  functions  of  Evolution,  namely,  Heredity  and 
Adaptation,  and  has  left  this  entirely  physiological  task  to 
morphologists.  We  owe  to  morphologists,  and  not  to  physi- 
ologists, nearly  all  that  we  yet  know  of  Heredity  and 
Adaptation.  The  latter  still  works  as  little  at  the  functions 
of  evolution  as  at  the  evolution  of  the  functions. 

It  will,  therefore,  be  the  task  of  a  future  Physiogeny  to 
grasp  the  history  of  the  evolution  of  the  functions  with  the 
same  earnestness,  and  with  the  same  success,  with  which 
Morphogeny  has  long  ago  undertaken  the  study  of  the  history 
of  the  evolution  of  forms.  A  few  instances  will  show  how 
closely  the  two  are  connected.  The  heart  of  the  human 
embryo  has  at  first  a  very  simple  structure,  such  as  appears 
permanently  only  in  Asc'dians  and  other  inferior  Worms, 
and  connected  with  it  is  a  circulation  of  the  blood  oi 
the  most  simple  kind.  When,  on  the  other  hand,  we  see 
that  with  the  fully  developed  form  of  the  human  heart  there 
is  connected  a  function  of  the  circulation  of  the  blood  totally 
different  from  the  former  one,  and  far  more  complicated,  the 
study  of  the  evolution  of  the  heart  necessarily  enlarges 
from  a  task  which  was  originally  morphological  to  one 
which  is  physiological  also.  It  is  the  same  in  the  case  oi 
all  other  organs  and  their  activities. 

Thus,  for  instance,  a  careful  comparative  study  of  the 
history  of  the  evolution  of  the  form  of  the  intestinal  canal, 
the  lungs,  and  the  organs  of  generation,  affords  us  also  most 
important  information  as  to  the  evolution  of  the  respective 
functions  of  these  organs. 


22  THE   EVOLUTION   OF   MAN. 

This  important  relation  is  most  clearly  seen  in  th« 
history  of  the  evolution  of  the  nervous  system.  In  the 
economy  of  the  human  body,  this  system  performs  the  func- 
tions of  sensation,  of  voluntary  movement,  volition,  and 
finally  the  highest  psychical  functions,  namely,  those  of 
thought ;  in  a  word,  every  one  of  the  various  activities  which 
constitute  the  special  subject  of  Psychology,  or  the  science 
of  the  mind.  Modern  Anatomy  and  Physiology  have  demon- 
strated that  these  functions  of  the  mind,  or  psychic  activities, 
are  immediately  dependent  upon  the  more  delicate  structure 
of  the  central  nervous  system,  upon  the  internal  conditions 
of  the  form  of  the  brain  and  the  spinal  marrow.  Here 
are  placed  the  extremely  complex  mechanism  of  cells,  whose 
physiological  function  constitutes  the  mind-life  of  Man. 
It  is  so  complex  that  to  most  people  its  function  appears 
to  be  something  supernatural,  and  incapable  of  mechanical 
explanation.  But  the  history  of  the  evolution  of  the  in- 
dividual furnishes  us  with  the  most  surprising  and  signi- 
ficant information  as  to  the  gradual  origin  and  progressive 
formation  of  this  most  important  system  of  organs.  For  the 
first  rudiment  of  the  central  nervous  system  in  the  human 
embryo  makes  its  appearance  in  the  same  most  simple  form 
in  which  Ascidians  and  other  inferior  Worms  retain  it 
throughout  life.  A  perfectly  simple  spinal  marrow,  without 
brain,  such  as  throughout  its  existence  represents  the  organ 
of  the  mind  of  the  Amphioxus,  the  lowest  of  Vertebrates, 
first  develops  from  this  rudiment.  It  is  only  at  a  later 
period  that  a  brain  develops  from  the  anterior  extremity 
of  this  spinal  cord,  and  this  brain  is  of  the  simplest  form, 
similar  to  the  permanent  form  of  this  organ  in  the  lower 
Fishes.  Step  by  step  this  simple  brain  develops  still 
further,  passing  through  forms  corresponding  to  those  of 


ONTOGENY  AND  PHYLOGENY.  23 

the  Amphibia,  Beaked  Animals  (Ornithostoma),  Pouched 
Animals,  or  Marsupials,  and  Semi-apes  (Prosimice),  until  the 
highly  organized  form  is  reached  which  distinguishes  the 
Apes  from  all  other  Vertebrates,  and  which  finally  attains 
its  highest  development  in  the  human  brain.  But  step  by 
step  with  this  progressive  evolution  of  the  form  of  the 
brain,  the  evolution  of  its  peculiar  function,  the  psychical 
activities,  moves  on  hand  in  hand,  and  it  is  therefore  the 
history  of  the  evolution  of  the  central  nervous  system  which 
for  the  first  time  enables  us  to  understand  the  origin  of  life 
of  the  human  mind  from  natural  causes,  and  the  gradual 
historic  development  of  the  psychic  activities  of  man.  It  i1* 
impossible  without  the  aid  of  Ontogeny  to  perceive  how 
these  highest  and  most  brilliant  functions  of  the  animal 
organism  have  been  historically  developed.  In  a  word,  the 
history  of  the  evolution  of  the  spinal  marrow  and  the  brain 
of  the  human  embryo  at  the  same  time  directly  leads  us 
to  understand  the  Phylogeny  of  the  human  mind,  that  most 
sublime  activity  of  life  which  in  the  developed  human  being 
we  are  accustomed  to  regard  as  something  wonderful  and 
supernatural. 

There  is  no  doubt  that  this  special  result  of  the  study 
of  the  history  of  evolution  is  among  the  greatest  and  most 
important.  Happily,  our  knowledge  of  the  Ontogeny  of  the 
central  nervous  system  of  Man  is  so  satisfactory,  and  agrees 
so  perfectly  with  the  supplementary  results  of  Comparative 
Anatomy  and  Physiology,  that  it  affords  us  a  perfectly 
clear  insight  into  one  of  the  highest  problems  of  philosophy, 
namely,  the  Phylogeny  of  the  psyche,  the  mind,  or  the 
history  of  the  ancestral  lineage  of  Man's  psychic  activities, 
and  leads  us  into  the  only  path  by  which  we  shall  ever  be 
able  to  solve  this  the  highest  of  all  problems. 


THE   EVOLUTION   OF   MAN. 


TABLE    I. 

LIST  of  the  principal  branches  of  BIOOENY,  or  the  HISTORY  OF  ORGAN i. 
EVOLUTION,  with  reference  to  the  four  chief  stages  of  Organic  In 
dividuality— Cell,  Organ,  Person,  and  Race.14 


First  branch  of  Biogeny, 
or  of  the  history  of  the 
evolution  of  organisms: 
GERM-HISTORY,  or  On- 
togeny (history  of  the 
development  of  the 
embryo  of  the  in- 
dividual organism). 


.  Germ-history  of 

Forms. 
(Morphogeny.) 


'1.  Germ-history  of  the  cells  (and  cytods) 
and  of  the  tissues  composed  of  the  cells 
llistogeny. 

2.  Germ-history  of  the  organs,  and  of  the 
systems  and  apparatus  composed  of  the 
organs.     Organogeny. 

3.  Germ-history   of  the   persons   (called 
"  the  history  of  the  evolution  of  bodily 
form ").    ISlastogeny. 

4.  Genn-hisfory   of   races  (or   of    social 
aggregates   composed   of   persons:    fa- 
milies, communities,  states,  etc.    Cor- 
viogeny. 


iThe  germ-history  of  the  functions  or  the 
history  of  the  development  of  vital 
activities  in  the  individual,  has  not  yet 
been  accurately  and  scientifically  in- 
vestigated. 


n. 

Second  branch  of  Biogeny, 
or  of  the  history  of  the 
evolution  of  organisms: 
TRIBAL  HISTORY,  or 
Phylogeny  (history  of 
the  palseontological  evo- 
lution of  organic 


3.  Tribal  history 
of  Forms. 


1.  Tribal  history  of  the  cells  (hardly  at- 
tempted  as  yet).    Histophyly. 

2.  Tribal  history  of  organs  (an  unrecog- 
nized main  object  of  comparative  ana- 
tomy).   Organophyly. 

3.  Tribal  history  of  persons  (an  unrecog- 
nized main  object  of  the  natural  system 
of  classification).    Blastophyly. 

4.  Tribal  history   of  races  (or  of  social 
aggregates   composed   of  persons :    fa- 
milies, communities,  states,  etc.    Cor- 

.      mophyly. 


0  tribal  history  of  the  functions,  or  ttii 
listory  of  the  palaeontological  develop, 
nent  of  vital  activities,  has,  in  the  case 
f  most  organisms,  not  yet  been  ex- 
mined.  In  the  case  of  man,  a  large 
art  of  the  history  of  culture  fcUls  unrleJ 
hi*  head. 


CHAPTEE   II. 
THE  EARLIER  HISTORY  OF   ONTOGENY. 

CASPAR  FRIEDRICH  WOLFF. 

The  Evolution  of  Animals  as  known  to  Aristotle. — His  Knowledge  of  the 
Ontogeny  of  the  Lower  Animals. —  Stationary  Condition  of  the  Scien- 
tific Study  of  Nature  during  the  Christian  Middle  Ages. — First  Awaken- 
ing of  Ontogeny  in  the  Beginning  of  the  Seventeenth  Century. — Fa- 
bricius  ab  Aquapendente. — Harvey. — Marcello  Malpighi. — Importance 
of  the  Incubated  Chick. — The  Theories  of  Pre-formation  and  Encase- 
ment (Evolution  and  Pre -delineation). — Theories  of  Male  and  Female 
Encasement. — Either  the  Sperm-animal  or  the  Egg  as  the  Pre-formed 
Individual. — Animalculists :  Leeuwenhoek,  Hartsoeker,  Spallanzani. — 
Ovnlists  :  Haller,  Leibnitz,  Bonnet. — Victory  of  the  Theory  of  Evolution 
owing  to  the  Authority  of  Haller  and  Leibnitz. — Caspar  Friedrich  Wolff. 
— His  Fate  and  Works. — The  Theoria  Generationis. — Ke-formation,  or 
Epigenesis.— The  History  of  the  Evolution  of  the  Intestinal  Canal.— 
The  Foundations  of  the  Theory  of  Germ-layers  (Four  Layers,  or  Leaves). 
—The  Metamorphosis  of  Plants.— The  Germs  of  the  Cellular  Theory. 
—Wolff's  Monistic  Philosophy. 

"He  who  wishes  to  explain  Generation  must  take  for  his  theme  the 
organic  body  and  its  constituent  parts,  and  philosophize  about  them ;  he 
must  show  how  these  parts  originated,  and  how  they  came  to  be  in  that  rela- 
tion in  which  they  stand  to  each  other.  But  he  who  learns  to  know  a  thing 
not  only  directly  from  its  phenomena,  but  also  its  reasons  and  causes  j  and 
who,  therefore,  not  by  the  phenomena  merely,  but  by  these  also,  is  compelled 
to  say  :  '  The  thing  must  be  so,  and  it  cannot  be  otherwise ;  it  is  necessarily 
of  such  a  character ;  it  must  have  such  qualities ;  and  it  is  impossible  for 
it  to  possess  others' — understands  the  thing  not  only  historically  but 
truly  philosophically,  and  he  has  a  philosophic  knowledge  of  it.  Our  own 


2(5  THE   EVOLUTION   OF  MAN. 

Theory  of  Generation  is  to  be  such  a  philosophic  comprehension  of  an  organic 
body,  very  different  from  one  merely  historical."— CASPAK  FEIEDEICH  WOLPJ 
(1764). 

IN  approaching  each  science  it  is,  in  several  respects,  pro- 
fitable to  glance  at  the  course  of  its  evolution  The  well- 
known  principle  that  "  whatever  has  come  into  being  can 
only  be  known  from  the  process  by  which  it  came  into 
being"  is  applicable  to  science.  By  tracing  its  gradual 
development,  we  shall  most  clearly  perceive  its  tasks  and 
aims.  We  shall  also  find  that  the  present  condition  of  the 
History  of  the  Evolution  of  Man,  with  all  its  peculiar  cir- 
cumstances, can  only  be  properly  understood  by  taking  into 
consideration  the  history  of  the  evolution  of  the  science 
itself.  The  examination  will  not  detain  us  long ;  for  the 
History  of  the  Evolution  of  Man  is  one  of  the  very  youngest 
of  the  Natural  Sciences.  This  is  equally  true  of  its  two 
divisions :  the  History  of  the  Germ,  or  Ontogeny,  and  the 
History  of  the  Tribe,  or  Phylogeny. 

Passing  over  such  most  ancient  germs  of  the  science  as 
are  found  in  classical  antiquity,  and  which  we  shall  have 
to  discuss  presently,  the  true  History  of  the  Evolution  of 
Man,  as  a  science,  really  begins  in  the  year  1759,  when 
Caspar  Friedrich  Wolff,  one  of  the  most  eminent  of  German 
naturalists,  published  his  Theoria  Generations.  This  was 
the  first  foundation-stone  for  a  true  history  of  animal 
germs.  In  1809,  exactly  fifty  years  later,  Jean  Lamarck 
published  the  Philosophic  Zoologique,  the  first  attempt  at  a 
History  of  Descent ;  and  in  1859,  another  half  century  later, 
appeared  Darwin's  work,  which  must  be  regarded  as  the 
first  to  give  a  scientific  basis  to  that  attempt.  But,  before 
carefully  examining  this  as  the  real  foundation  of  the 


ARISTOTLE   ON   DEVELOPMENT.  2J 

History  of  the  Evolution  of  Man,  we  must  rapidly  glance  at 
the  great  philosopher  and  naturalist  of  antiquity,  who,  in 
this  as  well  as  in  all  other  branches  of  research  in  Natural 
Science,  stands  quite  alone  for  a  period  of  more  than  two 
thousand  years.  This  was  Aristotle,  "the  Father  of  Natural 
History." 

Among  the  extant  writings  of  Aristotle  on  Natural 
History,  treating  of  various  aspects  of  biological  research, 
and  the  most  important  of  which  is  the  History  of  Animals, 
there  occurs  also  a  smaller  work,  specially  confined  to  the 
History  of  Evolution.  It  is  entitled  Peri  Zoon  Geneseos 
("On  the  Generation  and  Development  of  Animals").15 
This  work  is  of  great  interest,  if  merely  because  it  is  the 
most  ancient,  and  the  only  one  of  its  kind,  which  has 
reached  us  from  classical  antiquity  in  a  fairly  complete 
condition.  It  is  important  also  because,  like  others  of 
Aristotle's  writings  on  subjects  of  Natural  History,  it 
entirely  controlled  the  science  for  two  thousand  years.  The 
philosopher  was  a  careful  observer  and  an  ingenious 
thinker ;  yet,  while  his  importance  as  philosopher  has  never 
been  doubted,  his  merits  as  an  observant  naturalist  have 
only  lately  been  duly  appreciated.  Those  students  of 
Nature  who  have  lately  more  accurately  examined  his 
writings  on  Natural  History,  have  been  astonished  at  the 
mass  of  interesting  statements,  and  the  remarkable  observa- 
tions which  abound  in  them.  With  regard  to  the  History 
of  Evolution,  it  is  specially  noticeable  that  Aristotle  traced 
it  in  the  most  diverse  classes  of  animals,  and  that  he  was 
acquainted,  especially  in  connection  with  the  lower  animals, 
with  several  of  the  most  remarkable  facts  which  we  have 
re-discovered  only  towards  the  middle  of  the  present 
century.  K 


28  THE   EVOLITIOX    OF   MAN. 

It  is  certain,  for  example,  that  he  was  thoroughly 
acquainted  with  the  entirely  peculiar  method  of  propagation 
and  development  of  the  Cuttle-fishes,  or  Cephalapods,  the 
embryo  of  which  has  a  bag  of  yelk  protruding  from  the 
mouth.  He  knew,  also,  that  embryos  of  Bees  can  be 
developed  from  the  egg  even  when  it  has  not  been  fertilized. 
The  so-called  parthenogenesis,  or  virginal  generation,  of 
Bees  has  been  proved  in  our  days  only  lately  by  the 
meritorious  zoologist,  Siebold,  of  Munich,  who  also  showed 
that  male  Bees  develop  from  unimpregnated,  and  female 
bees  only  from  impregnated  eggs.16  Aristotle  further 
relates  that  some  Fishes  (of  the  species  Serranus)  are 
hermaphrodites,  inasmuch  as  each  individual  has  male 
and  female  organs,  and  impregnates  itself.  This  fact,  also, 
has  only  lately  been  established.  He  also  knew  that  the 
embryos  of  several  species  of  Sharks  are  connected  with 
the  mother's  womb  by  a  sort  of  placenta — an  organ  of 
nourishment,  full  of  blood,  which  otherwise  occurs  only 
in  Man  and  the  higher  Mammals.  This  placenta  of  the 
Shark  was  for  a  long  time  considered  mythical,  until,  in 
1839,  Johannes  Muller,  of  Berlin,  proved  it  to  be  a  fact. 
We  might  quote  many  other  remarkable  observations  from 
Aristotle's  account  of  Evolution,  which  would  prove  the 
accuracy  of  this  great  naturalist's  acquaintance  with  onto- 
genetic  investigations,  and  the  great  degree  in  which  he 
was  in  advance  of  subsequent  times  in  this  respect. 

In  most  of  his  observations  he  was  not  satisfied  with 
merely  stating  the  facts,  but  he  added  reflections  on  their 
significance.  Some  of  these  theoretical  thoughts  are  of 
special  interest,  because  they  indicate  a  right  fundamental 
perception  of  the  nature  of  the  processes  of  evolution.  He 


ARISTOTLE   AS   A   NATURALIST.  29 

conceives  the  evolution  of  the  individual  to  be  a  new 
formation,  in  which  the  several  parts  of  the  body  develop 
one  after  the  other.  According  to  him,  when  the  human 
or  animal  individual  develops,  either  within  the  mother's 
body  or  out  of  it  in  the  egg,  the  heart  is  formed  first,  and 
is  the  beginning  and  the  centre  of  the  body.  After  the 
heart  has  been  formed,  the  other  organs  appear ;  of  these 
the  interior  precede  the  exterior,  and  the  upper,  or  those 
above  the  diaphragm,  precede  the  lower,  or  those  below  it. 
The  brain  is  formed  at  a  very  early  stage,  and  out  of  it 
grow  the  eyes.  This  assertion  is,  indeed,  quite  accurate.  On 
trying  to  obtain  from  these  statements  of  Aristotle  an  idea 
of  his  conception  of  the  processes  of  evolution,  we  find  that 
they  indicate  a  faint  presentiment  of  that  theory  of  evolution 
which  is  now  called  Epigenesis,  and  which  Wolff,  some  two 
thousand  years  later,  first  proved.  It  is  especially  remark- 
able that  Aristotle  altogether  denied  the  eternity  of  the 
individual.  He  admitted  that  the  kind  or  species,  formed 
from  individuals  of  the  same  kind,  might  possibly  be 
eternal ;  but  asserted  that  the  individual  itself  was  tran- 
sient, that  it  came  into  being  anew  in  the  act  of  genera- 
tion, and  perished  at  death. 

During  the  two  thousand  years  after  Aristotle  no 
essential  progress  in  Zoology  in  general,  or  in  the  History  of 
Evolution  in  particular,  is  to  be  recorded.  People  were 
content  to  expound  Aristotle's  zoological  writings,  to  copy 
them,  to  deface  them  greatly  by  additions,  and  to  translate 
them  into  other  languages.  There  was  hardly  any 
independent  research  during  this  long  period.  During  the 
Middle  Ages  of  Christianity,  when  insurmountable  obstacles 
were  laid  in  the  way  of  independent  researches  in 


JO  THE  EVOLUTION   OF   MAN. 

natural  science  by  the  development  and  diffusion  of 
influential  conceptions  of  faith,  a  re-commencement  of 
biological  researches  was  especially  out  of  the  question. 
Even  when,  in  the  sixteenth  century,  human  Anatomy 
again  began  to  be  studied,  and  independent  investigations 
of  the  structure  of  the  body  of  the  developed  human  being 
were  again  first  made,  anatomists  dared  not  extend  their 
investigations  into  the  condition  of  the  yet  undeveloped 
human  body,  into  the  formation  and  development  of  the 
embryo. 

The  prevailing  fear  of  such  researches  was  due  to 
several  causes.  This  seems  but  natural  when  we  remember 
that  by  the  bull  of  Pope  Boniface  VIII.  greater  excom- 
munication was  pronounced  against  all  who  dared  to  dis- 
member a  human  corpse.  While  anatomical  investiga- 
tion of  the  developed  human  body  was  a  crime  which 
drew  down  the  curse  of  the  Church,  it  is  evident  that  the 
examination  of  the  body  of  the  child,  hidden  in  the 
mother's  womb,  and  which  the  Creator  himself  seemed, 
by  its  concealed  position,  to  have  intentionally  withdrawn 
from  the  curious  gaze  of  naturalists,  would  have  appeared 
much  more  criminal  and  impious.  The  omnipotence  of 
the  Christian  Church,  which  at  that  time  caused  many 
thousands  to  be  executed  and  burned  for  heresy,  and  which 
even  then  with  correct  instinct  foresaw  danger  threatened 
to  itself  from  the  deadly  enemy  which  was  then  growing 
up  in  Natural  Science,  took  care  that  the  latter  should 
not  make  too  rapid  strides. 

It  was  only  when  the  Reformation  broke  the  all- 
embracing  power  of  the  Only-Saving  Church,  and  a  new 
and  fresh  intellectual  impulse  began  to  release  enslaved 


EARLIER   STUDENTS   OF   EVOLUTION.  31 

science  from  the  iron  chains  of  dogmatism,  that  human 
Anatomy  and  the  History  of  the  Evolution  of  Man  could 
move  again  more  freely,  with  the  re-opening  of  research  in 
other  natural  sciences.  But  Ontogeny  remained  far  behind 
Anatomy,  and  it  was  only  in  the  beginning  of  the  seven- 
teenth century  that  the  first  ontogenetic  publications 
appeared.  The  first  to  begin  was  the  Italian  anatomist, 
Fabricius  ab  Aquapendente,  Professor  at  Padua,  who  pub- 
lished two  works — De  Formato  Foetu  (1600),  and  De 
Formations  Fo3tus  (1604), — which  contain  the  oldest 
figures  and  descriptions  of  the  embryo  of  Man  and  other 
Mammals,  and  also  of  the  Chick.  Similar  imperfect 
representations  were  given  soon  after  by  Spigelius — De 
Formato  Fcetu  (1631) — by  the  Englishman,  Needham 
(1667),  and  his  celebrated  countryman,  Harvey  (1652).  The 
latter  discovered  the  circulation  of  the  blood  in  the  animal 
body,  and  made  the  important  assertion :  Omne  vivum  ex 
ovo  ("Everything  living  comes  from  an  egg").  The  Dutch 
naturalist,  Swammerdam,  in  his  "  Bible  of  Nature,"  pub- 
lished the  results  of  the  first  investigations  into  the 
embryology  of  the  Frog,  and  the  so-called  segmentation  of 
its  yelk.  The  most  important  ontogenetic  researches  of  the 
seventeenth  century,  however,  were  those  of  the  Italian, 
Marcello  Malpighi  of  Bologna,  who  gave  a  fresh  impetus 
both  to  Zoology  and  to  Botany.  His  two  dissertations,  De 
Formatione  Pulli,  and  De  Ovo  Incubato  (1687),  contain  the 
first  connected  description  of  the  history  of  the  development 
of  the  chick  in  the  incubated  egg. 

Here  I  must  make  some  remarks  on  the  great  importance 
of  the  Chick  in  relation  to  our  science.  The  history  of  the 
formation  of  a  Chick,  as  well  as  of  all  birds,  accurately 


j2  THE   EVOLUTION   OF   MAN. 

corresponds  in  its  essential  characteristics  with  that  ^of  all 
other  higher  Vertebrates;  and,  therefore,  also  of  Man. 
The  three  higher  classes"  of  Vertebrates,  Mammals,  Birds, 
and  Reptiles  (Lizards,  Snakes,  Turtles,  etc.),  are  from  the 
beginning  of  their  individual  development  so  surprisingly 
similar  in  all  essential  features  of  their  bodily  structure, 
especially  in  the  earlier  stages,  that  for  a  long  while  it  is 
impossible  to  distinguish  them.  (Of.  Plates  VI.  and  VII.) 
It  has  long  been  known  that  the  accurate  study  of  the 
evolution  of  the  embryo  of  the  Bird,  which  is  most  readily 
obtained  as  the  subject  of  research,  is  all  that  is  necessary 
in  order  to  learn  the  essentially  similar  mode  of  evolution 
of  Mammals,  therefore  also  of  Man.  Even  as  early  as  the 
middle  and  the  end  of  the  seventeenth  century,  when 
human  embryos,  as  well  as  those  of  all  other  Mammals, 
began  to  be  examined  in  their  earlier  stages,  this  most 
important  fact  was  soon  recognized.  It  is  of  the  greatest 
importance,  both  for  theoretical  and  for  practical  purposes. 
Conclusions  of  the  highest  importance  to  the  theory  of 
evolution  may  be  drawn  from  the  similarity  of  structure 
of  the  embryos  of  widely  differing  animals.  This  simi- 
larity is  invaluable  in  practical  ontogenetic  research, 
because  the  ontogeny  of  Birds,  which  is  accurately  known, 
most  completely  supplements  and  explains  the  embryology 
of  Mammals,  which  has  been  but  imperfectly  studied. 
Hen's  eggs  can  be  obtained  at  all  times  and  in  any  quan- 
tity, and  by  hatching  them  artificially  the  evolution  of 
the  embryo  may  be  traced  step  by  step.  On  the  other 
hand,  it  is  much  more  difficult  to  study  the  evolution  of 
Mammals,  because  the  embryo  of  these  does  not  develop 
in  a  large  egg  that  has  been  laid,  or,  in  other  words,  in  an 


IMPORTANCE   OF   THE   CHICK.  33 

independent  and  isolated  body,  but  in  a  small  egg,  which, 
until  maturity,  remains  enclosed  and  concealed  in  the  body 
of  the  mother.  For  this  reason  it  is  very  difficult  to  pro- 
cure all  the  stages  of  development  in  any  large  number, 
for  the  purpose  of  making  connected  investigations,  not 
to  mention  external  reasons,  such  as  the  great  cost,  the 
technical  difficulties,  and  the  many  other  obstacles,  which 
lie  in  the  way  of  any  extended  series  of  researches  into 
fecundated  mammals.  For  this  reason,  from  that  time  to 
the  present  day,  the  Chick  during  the  process  of  incubation 
has  been  the  subject  oftenest  and  most  closely  investigated. 
The  perfection  of  hatching-machines  has  made  it  yet  easier 
to  obtain  embryo-chicks  in  any  required  stage  of  evolution 
and  in  any  quantity,  in  order  to  examine  the  whole  process 
of  formation  step  by  step. 

About  the  end  of  the  seventeenth  century  the  history  of 
the  evolution  of  the  incubated  Chick  had  already  been 
advanced  as  far,  and  its  more  essential,  external,  and  less 
delicate  conditions  were  as  well  known,  owing  to  the 
labours  of  Malpighi,  as  investigations  with  the  imperfect 
microscopes  of  that  time  rendered  possible.  Of  course,  the 
perfection  of  the  microscope  and  of  technical  methods  of 
research  was  a  necessary  condition  for  more  accurate  em- 
bryological  research.  For  vertebrate  embryos  in  their 
earlier  stages  are  so  small  and  delicate,  that  it  is  impossible 
to  examine  them  without  a  good  microscope,  and  without 
applying  peculiar  technical  methods.  But  these  means 
were  not  applied,  and  the  microscope  was  not  essentially 
perfected  till  the  beginning  of  our  century. 

Throughout  the  whole  of  the-  first  half  of  the  eighteenth 
century,  during  which  time  the  systematic  Natural  History 


34  THE   EVOLUTION   OF   MAN. 

of  animals  and  plants  received  so  great  an  impulse  from 
Linnaeus'  famous  Systema  Naturae,  the  History  of  Evolution 
made  scarcely  any  progress.  It  was  in  the  year  1759  that 
Caspar  Friedrich  Wolff  made  his  appearance,  and  his  genius 
gave  an  entirely  new  direction  to  this  science.  Until  then 
Embryology  was  almost  exclusively  occupied  in  unsuccessful 
attempts  to  construct  various  theories  of  evolution  from  the 
scanty  material  already  acquired. 

The  theory  which  at  that  time  gained  almost  universal 
acceptance,  and  which  continued  to  be  generally  received 
during  the  entire  eighteenth  century,  is  in  Germany  com- 
monly called  the  Theory  of  Unfolding  (Auswickelung),  or 
Evolution,  but  is  better  spoken  of  as  the  Theory  of  Pre- 
formation.17  Its  main  idea  is  the  following  :  no  really  new 
formation  takes  place  during  the  evolution  of  each  indi- 
vidual organism,  animal  or  plant,  including  therefore  Man  : 
there  is  only  a  growth  and  an  unfolding  of  parts,  all 
of  which  have,  from  eternity,  been  present,  pre-formed,  and 
complete,  though  only  very  minute,  and  wrapped  together. 
Every  organic  germ,  therefore,  contains  all  the  parts  and 
organs  of  the  body  pre-formed  and  represented  in  their 
subsequent  form,  position,  and  connection,  and  the  entire 
course  of  the  evolution  of  the  individual,  the  entire  onto- 
genetic  process,  is  nothing  but  an  evolution  in  the  most 
exact  meaning  of  the  word;  namely,  an  unwrapping  of 
wrapped-up  parts  already  formed.  Hence,  for  example,  in 
a  hen's  egg  we  do  not  find  a  simple  cell  which  undergoes 
division,  and  the  generation  of  cells  of  which  form  layers  of 
germs,  and  by  various  changes,  separations,  and  new  for- 
mations, ultimately  bring  into  being  the  body  of  the  Bird ; 
but  every  hen's  egg  contains  from  the  beginning  a  complete 


THEORIES   OF    PRE -FORMATION   AND   ENCASEMENT.  35 

Chick,  with  all  its  parts  pre-formed  and  wrapped  together, 
and  during  the  development  of  the  incubated  egg  these 
parts  are  merely  drawn  out  and  grow. 

As  soon  as  this  theory  was  carried  out  logically,  it 
necessarily  led  to  the  Theory  of  Encasement.  According  to 
this,  every  species  of  animal  or  plant  was  originally  creatt3d 
only  as  a  pair  or  as  a  single  individual ;  but  this  one  indi- 
vidual already  contained,  encased  within  itself,  the  germs  of 
all  the  other  individuals  of  its  species  which  have  ever  lived 
or  will  live.  As  at  that  time  the  age  of  the  earth  was 
calculated,  according  to  the  Biblical  history  of  creation,  at 
five  or  six  thousand  years,  people  thought  they  could 
approximately  calculate  the  number  of  germs  of  every 
species  of  organism  which  had  lived  during  that  period,  and 
consequently  the  number  which  had  existed  encased  in  the 
first  "  created  "  individual  of  the  species.  The  theory  was 
logically  extended  to  mankind,  and  it  was  accordingly 
maintained  that  our  first  common  mother  Eve  held  in  her 
ovary  the  germs  of  all  the  children  of  men,  one  encased  in 
the  other. 

This  Theory  of  Encasement  was  then  developed  so  that 
the  female  individuals,  were  considered  to  be  the  created 
beings  which  were  encased  one  in  another.  It  was  believed 
that  only  a  single  pair  of  each  species  was  originally 
created ;  but  the  ovary  of  the  female  individual  contained, 
encased  within  it,  alJ  the  germs  of  all  the  individuals  of 
the  kind,  of  both  sexes,  which  were  ever  to  develop.  But 
the  Theory  of  Pre-formation  took  quite  another  shape  when, 
in  1690,  Leeuwenhoek,  the  Dutch  microscopist,  discovered 
the  human  spermatozoids,  or  seminal  threads,  and  proved 
that  a  large  number  of  extremely  delicate  and  actively 


36  THE   EVOLUTION   OF   MAN. 

moving  threads  exist  in  the  sperm  or  seminal  fluid  of  the 
male.  (Cf.  Fig.  17  in  Chap.  VII.)  This  astonishing  discovery 
was  at  once  interpreted  to  the  effect  that  these  minute 
living  bodies,  briskly  swimming  about  in  the  seminal  fluid, 
were  genuine  animals,  the  pre-formed  germs  of  future 
generations.  When  at  the  time  of  fecundation  the  two 
generative  substances,  male  and  female,  came  in  contact 
with  each  other,  these  thread-like  seminal  animalcules  were 
to  penetrate  into  the  fruitful  soil  of  the  ovary  and  there  to 
attain  their  development  like  vegetable  seeds  in  the  fruitful 
soil  of  the  earth.  According  to  this  theory  every  single 
seminal  animalcule  of  Man  is  a  complete  human  being;  all 
the  separate  parts  of  the  body  would  be  entirely  pre-formed 
in  it,  and  subject  only  to  a  mere  unwrapping  and  enlargement 
as  soon  as  they  reached  the  favourable  matrix  of  the  female 
egg.  This  theory  also  was  logically  carried  out  to  the  effect 
that  in  every  single  thread-like  body  were  contained  all  the 
subsequent  generations  of  its  descendents,  one  encased  in 
the  other,  each  in  the  most  extreme  degree  of  fineness,  and 
of  the  minutest  size.  The  seminal  gland  of  Adam,  therefore, 
contained  the  germs  of  all  the  children  of  men  who  have 
ever  peopled  our  planet,  who  inhabit  it  at  present,  or  will 
occupy  it  in  the  future  "  until  the  end  of  the  world." 

Of  course,  this  Doctrine  of  Encasement  in  the  Male  was 
utterly  opposed  to  the  Doctrine  of  Encasement  in  the  Female, 
which  had  previously  prevailed.  The  only  ground  common, 
to  both  was  the  false  idea  that  the  germs  of  innumerable 
generations,  previously  formed  and  encased  one  in  another, 
existed  in  every  organism ;  a  conception  on  which  was  also 
founded  the  curious  Prolepsis  Theory  of  Linnseus. 

The  two  opposite  theories  of  encasement  soon  began  a 


ANIMALCULISTS   V.    OVULISTS.  37 

vigorous  contest,  "which  resulted  in  the  division  of  the 
physiologists  of  the  eighteenth  century  into  two  large 
bodies  of  combatants,  entirely  opposed  and  contending 
vehemently.  These  were  the  Animalculists,  and  the  Ovn- 
lists.  The  dispute  between  these  two  parties  appears 
laughable  to  us  now,  for  the  theory  of  the  one  is  just  as 
unfounded  as  that  of  the  other.  As  Alfred  Kirchhoff  says, 
in  an  excellent  biographical  sketch  of  Wolff,  "  this  dispute 
was  as  little  capable  of  settlement,  as  the  inquiry  whether 
the  angels  lived  in  the  East  or  in  the  West  of  the  heavenly 
rogions." 18 

The  Animalculists,  or  the  Believers  in  Sperm,  looked 
upon  the  moving  seminal  threads  as  the  real  animal  germs, 
and  they  found  support  on  the  one  hand  in  the  lively 
movement,  and  on  the  other  in  the  form  of  these  seminal 
animalcules.  For  in  the  case  of  man,  as  well  as  of  a  large 
majority  of  other  animals,  they  appear  to  have  a  somewhat 
oblong,  egg-like,  or  pear-like  head,  a  thin  intermediate 
segment,  and  a  very  thin  tail,  narrowing  to  a  hair-like 
form  (Fig.  17).  In  reality,  the  whole  formation  is  but  a 
simple  whip-shaped  cell.  The  head  is  the  cellular  nucleus, 
surrounded  by  cell-matter,  which  is  protracted  into  the 
thinner  portions  in  the  middle,  and  to  the  hair-like,  move- 
able  iail ;  the  latter  is  the  whip,  or  thread-like  appendage  of 
other  whip-shaped  cells.  The  Animalculists,  however,  con- 
Bidered  the  head  to  be  a  real  animal  head,  and  the  rest  of 
the  body  to  be  a  complete  animal  body.  Leeuwenhoek, 
Haitsoeker,  and  Spallanzani  were  the  chief  defenders  of 
this  theory  of  Pre-delineation. 

The  opposite  party,  the  Ovulists  (Ovists),  or  Believers 
in  ^ggs,  who  adhered  to  the  older  Theory  (if  Evolution, 


3  8  THE   EVOLUTION   OF   MAN. 

maintained  that  the  egg  was  the  real  animal  germ,  and 
that  the  seminal  animalcules,  at  the  time  of  fecundation, 
only  gave  the  impulse  which  caused  the  unfolding  of  the 
eo-g  in  which  all  generations  were  encased  one  in  the  other. 
This  opinion  prevailed  with  the  majority  of  biologists 
during  the  whole  of  the  last  century,  though  Wolff,  in 
1759,  demonstrated  its  utter  want  of  foundation.  Its 
acceptance  was  specially  due  to  the  fact  that  the  most 
celebrated  biological  and  philosophical  authorities  of  that 
time  had  pronounced  in  its  favour, — among  them  princi- 
pally Haller,  Bonnet,  and  Leibnitz. 

Albrecht  Haller,  Professor  at  Gottingen,  who  has  often 
been  called  "  the  Father  of  Physiology,"  was  a  very  learned 
and  comprehensively  educated  man,  but,  as  an  interpreter 
of  the  more  profound  natural  phenomena,  occupied  no 
very  high  position.  He  has  best  described  himself  in  the 
celebrated  and  often-cited  saying,  that  "  Into  the  inner  side 
of  Nature  no  created  mind  ever  penetrates;  happy  he  to 
whom  she  shows  only  her  outer  husk  ! "  The  best  answer 
to  this  "  husk  "  view  of  nature  was  given  by  Goethe,  in  hi* 
splendid  poem  which  ends  with  the  lines : 

"  Nor  hnsk  nor  kernel  Nature  brings — 

For  all  one  only  type  of  things  ; 
.  Yet  prove  thyself,  and  seek  to  know 

If  husk  or  kernel  thou  dost  show." 

Attempts  have,  however,  been  recently  made  to  justi  fy 
Haller's  "  husk  "  view.  Wilhelm  His  has  made  himself  the 
special  defender  of  this  strange  conception.  Haller,  in  his 
well-known  work,Elementa  Physiologies,  adopted  the  Theory 
of  Evolution  (Theory  of  Pre-formation)  in  a  most  decided 
manner,  in  these  words  :  "  There  is  no  coming  into  being  J 


HALLER   AND   LEIBNITZ.  39 

(Nulla  est  epigenesis).  No  part  of  the  animal  body  was  made 
previous  to  another,  and  all  were  created  simultaneously 
(Nulla  in  corpore  animali  pars  ante  aliam  facto,  est, 
et  omnes  simul  creates  existunfy."  In  reality,  therefore, 
he  denied  any  actual  evolution  in  the  natural  sense,  and 
in  this  went  so  far  as  to  maintain  even  the  existence  of  a 
beard  in  the  new-born  boy,  and  the  existence  of  the  horns 
in  the  hornless  fawn ;  all  the  parts  were  already  present 
in  a  complete  state,  but  hidden  for  a  while  from  the  human 
eye.  Haller  even  calculated  the  number  of  human  beings 
which  God,  on  the  sixth  day  of  His  work  of  creation,  at 
once  created  and  encased  in  the  ovary  of  Eve,  the  Mother 
of  all.  He  estimated  them  at  two  hundred  thousand 
millions,  by  assuming  the  creation  of  the  world  to  have 
been  six  thousand  years  ago,  the  average  human  life  thirty 
years,  and  the  number  of  human  beings  alive  at  the  same 
time  one  thousand  million.  And  the  celebrated  Haller 
advocated  all  this  rampant  nonsense,  and  the  inferences 
drawn  from  it,  most  successfully,  even  after  Wolff  had  dis- 
covered the  true  Epigenesis,  and  proved  it  by  investigation. 
Leibnitz  was  the  most  important  of  the  philosophers 
who  adopted  the  Theory  of  Evolution  (Pre-formation),  and 
by  his  great  authority,  as  well  as  by  his  talented  exposition, 
gained  numerous  followers  for  it.  Based  upon  his  Theory 
of  Monads,  according  to  which  soul  and  body  are  in  an 
eternally  inseparable  union,  and  in  their  bi-unity  constitute 
Ihe  individual  (the  Monad),  Leibnitz  quite  logically  applied 
the  Theory  of  Encasement  to  the  soul  also,  and  denied  all 
real  development  for  it,  equally  with  the  body.  In  his 
Theodicce,  for  instance,  he  says :  "  I  think  that  souls,  which 
will  some  day  be  human  souls,  as  in  the  case  of  those  of 


^O  THE   EVOLUTION   OF   MAN. 

other  species,  pre-existed  in  the  semen ;  that  they  existed 
in  the  ancestors  as  far  back  as  Adam,  therefore  since  the 
beginning  of  things,  always  in  the  form  of  organized  bodies." 

The  Theory  of  Encasement  seemed  to  receive  its  most 
important  experimental  support  in  the  researches  of  Bonnet 
one  of  its  most  zealous  adherents.  He  observed,  for  the 
first  time,  in  Plant-lice,  the  so-called  "  virginal  generation," 
or  parthenogenesis,  which  is  an  interesting  form  of  propaga- 
tion lately  proved  by  Siebold  and  others,  in  many  other 
articulated  animals,  such  as  various  Crabs  and  Insects.16 
The  females  of  these  and  other  lower  animals  of  certain 
groups  propagate  for  several  generations  without  having 
been  impregnated  by  a  male.  Such  eggs,  which  for  their 
evolution  do  not  require  to  be  impregnated,  are  called 
"  false  eggs,"  Pseudova,  or  Spores.  Bonnet,  in  1745,  for  the 
first  time  observed  that  a  female  Plant-louse,  which  he  had 
completely  shut  off,  as  in  a  nunnery,  and  shielded  from  all 
contact  with  males,  after  shedding  its  skin  four  times,  gave 
birth  on  the  eleventh  day  to  a  living  female,  and  within 
the  next  twenty  days  produced  as  many  as  ninety-four 
other  females ;  and  that  soon  all  of  these,  without  having 
come  in  contact  with  a  male,  multiplied  again  in  the  same 
virgin  manner.  Thereupon,  of  course,  it  seemed  that  a 
tangible  proof  of  the  truth  of  the  Theory  of  Encasement, 
according  to  the  interpretation  of  Ovulists,  had  been 
Abundantly  furnished,  and  it  naturally  became  almost  uni- 
versally accepted  in  this  sense. 

The  case  stood  thus,  when  suddenly,  in  the  year  1759, 
'Jaspar  Friedrich  Wolff,  then  a  young  man,  appeared,  and 
with  his  new  Theory  of  Epigenesis  gave  the  death-blow  to 
the  entire  Theory  of  Pre-formation.  Wolff  was  born  at 


CASPAR   FRIEDRIUH   WOLFF.  41 

Berlin,  in  1733.  He  was  the  son  of  a  tailor,  and  studied 
natural  science  and  medicine  at  first  in  Berlin,  at  the 
Medico-surgical  College,  under  the  celebrated  anatomist 
Meckel,  and  subsequently  in  Halle.  Here,  in  the  twenty- 
sixth  year  of  his  age,  he  passed  his  examination  for  his 
doctor's  degree ;  and  on  the  28th  of  November,  1759,  in  his 
dissertation  as  doctor,  he  defended  the  new  doctrine  of  true 
evolution,  the  Theoria  Generationis,  founded  on  Epigenesis. 
This  dissertation,  in  spite  of  its  small  limits  and  difficult 
language,  ranks  among  the  most  important  essays  ever 
written  in  the  whole  range  of  biological  literature.  It  is 
equally  distinguished  by  its  abundance  of  new  and  most 
careful  researches,  and  by  its  far-reaching  and  very  sug- 
gestive ideas  given  in  connection  with  the  observations, 
which  latter  he  developed  into  a  brilliant  Theory  of  Evolu- 
tion entirely  true  to  nature.  Yet  this  remarkable  publica- 
tion had  at  first  no  results  whatever.  Although  the  study 
of  Natural  Science  was  then  flourishing  in  consequence  of 
the  impetus  given  by  Linnaeus;  although  botanists  and 
zoologists  soon  numbered,  not  dozens,  but  hundreds;  yet 
hardly  anybody  took  any  interest  in  Wolffs  Theory  of 
Generation.  And  the  few  who  had  read  it,  foremost  among 
whom  was  Haller,  considered  it  totally  false. 

Although  Wolff  proved  the  truth  of  Epigenesis  by 
means  of  the  most  accurate  research,  and  refuted  the  un- 
founded hypotheses  of  the  Theory  of  Pre-formation,  yet 
the  "exact"  physiologist  Haller  continued  to  be  the  most 
zealous  adherent  of  the  latter,  and  rejected  the  correct 
doctrine  of  Wolff  with  his  dictatorial  decree :  Nulla  est 
epigenesis !  It  is  not  surprising  that  the  entire  body  of 
physiological  scholars  of  the  second  half  of  the  eighteenth 


4.2  THE   EVOLUTION   OF   MAN. 

century  submitted  to  the  dictum  of  this  physiological  pope, 
and  opposed  Epigenesis  as  a  dangerous  innovation.  More 
than  half  a  century  elapsed  before  Wolff's  labours  met  with 
their  deserved  acknowledgment.  Only  after  Meckel,  in  the 
year  1812,  had  translated  into  German  another  most  im- 
portant publication  of  Wolffs,  "  On  the  Formation  of  the 
Intestinal  Canal "  (published  1764),  and  had  drawn  atten- 
tion to  its  extraordinary  significance,  people  began  to  re- 
occupy  themselves  with  this  almost  forgotten  author,  who, 
of  all  the  naturalists  of  the  preceding  century,  had  made  the 
deepest  progress  into  the  knowledge  of  the  living  organism. 

Thus,  as  so  often  happens  in  the  history  of  human  know- 
ledge, new-born  truth  succumbed  to  all-powerful  error, 
upheld  by  the  weight  of  authority.  The  knowledge  of  Epi- 
genesis, clear  as  the  sun,  was  not  able  to  pierce  through  the 
thick  fog  of  the  Dogma  of  Pre-formation,  and  its  ingenious 
discoverer  was  vanquished  in  the  fight  for  the  truth  by  the 
overwhelming  power  of  the  enemy. 

The  result  was  that  all  progress  in  the  History  of  Evo- 
lution was  for  a  while  arrested.  This  is  all  the  more  to  be 
regretted  because  Wolff  was  finally  compelled,  by  untoward 
circumstances,  to  quit  his  German  Fatherland.  From  the 
first  without  means,  he  had  only  been  able  to  finish  his  clas- 
sical work  in  the  face  of  great  difficulties,  and  was  then  com- 
pelled to  earn  his  bread  as  a  practising  physician.  During 
the  Seven  Years'  War  he  was  busy  in  the  Silesian  hospital?, 
and  gave  excellent  lectures  on  Anatomy  in  the  field  hospital 
of  Breslau,  attracting  the  attention  of  Cothenius,  the 
eminent  Director  of  Hospitals.  When  peace  had  been  con- 
cluded, this  distinguished  patron  tried  to  procure  a  chaii 
in  Berlin  for  Wolff,  but  failed  on  account  of  the  narrow- 


HISTORY    OF   WOLFF.  43 

mindedness  of  the  professors  of  the  Berlin  Medico-surgical 
College,  who  were  averse  to  all  scientific  progress.  This 
most  learned  faculty  persecuted  the  Theory  of  Epigenesis 
as  one  of  the  most  dangerous  heresies ;  just  as  is  the  case 
now  with  the  Theory  of  Descent.  Although  Cothenius, 
and  other  patrons  in  Berlin,  took  a  warm  interest  in  Wolff, 
it  was  impossible  even  to  procure  permission  for  him  to 
give  public  lectures  on  Physiology  in  Berlin.  The  conse- 
quence was,  that  Wolff  was  obliged  to  accept  a  summons 
with  which  the  Empress  Catherine  of  Russia  honoured  him 
in  17G6.  He  went  to  St.  Petersburg,  where  he  remained 
for  twenty-seven  years,  devoting  himself  in  undisturbed 
quiet  to  his  deep  researches,  and  enriching  the  publications 
of  the  St.  Petersburg  Academy  with  the  productions  of  his 
brilliant  talents.  He  died  there  in  1794.19 

The  progress  which  Wolff  made  in  the  entire  science  of 
Biology  was  so  great  that  the  naturalists  of  that  time  could 
not  grasp  it.  The  mass  of  important  new  researches,  and 
of  fruitful  and  great  ideas  accumulated  in  his  publications, 
is  so  enormous  that  their  full  value  has  only  been  gradually 
appreciated,  and  their  bearing  properly  understood  during 
the  present  century.  Wolff  opened  up  the  right  path  into 
the  most  various  branches  of  biological  investigations. 
Firstly,  and  above  all,  by  the  Theory  of  Epigenesis,  he 
first  made  the  real  nature  of  organic  evolution  intelligible. 
lie  proved  satisfactorily  that  the  evolution  of  every  organ- 
ism consists  of  a  series  of  new  formations,  and  that  no 
trace  of  the  form  of  the  developed  organism  exists  either  in 
the  egg  or  in  the  semen  of  the  male.  These  are  simple 
bodies  of  an  entirely  different  significance.  The  germ,  or 
embryo  which  develops  from  the  egg,  shows  in  the  various 


44  THE   EVOLUTION   OF   MAN. 

phases  of  its  evolution  an  internal  structure  and  an  external 
form  totally  different  from  those  of  the  developed  organism. 
In  none  of  these  phases  do  we  find  any  pre-formed  parts ; 
nowhere  any  encasement.  In  these  days  we  can  scarcely 
continue  to  call  this  Theory  of  Epigenesis  a  theory,  for  NQ 
have  been  thoroughly  convinced  of  its  correctness  in  fact, 
and  we  are  able  to  demonstrate  it  in  any  moment  under  the 
microscope.  Nor,  during  the  last  decade,  has  any  doubt  of 
the  truth  of  Epigenesis  been  expressed. 

Wolff  supplied  detailed  proof  of  his  Theory  of  Epigenesis 
in  his  scholarly  treatise  "On  the  Formation  of  the  Intestinal 
Canal  (1768)."  In  its  complete  condition  the  intestinal 
canal  of  the  Chick  is  a  very  complex,  long  tube,  to  which 
the  lungs,  the  liver,  the  salivary,  and  many  smaller  glands 
are  attached.  Wolff  showed  that  there  is  no  trace  of  this 
complex  tube,  with  all  its  various  parts,  in  the  embryo 
Chick  during  the  first  period  of  incubation,  but  that  in  its 
place  there  is  a  flat,  leaf-shaped  body  j  and  that  the  whole 
embryo-body  in  the  earliest  period  is  also  of  a  flat,  oblong, 
leaf-like  form.  Considering  the  difficulty  of  accurately  ex- 
amining conditions  so  extremely  minute  and  delicate  as  the 
first  leaf-shaped  beginnings  of  the  body  of  the  bird  with  the 
indifferent  microscopes  of  the  last  century,  we  cannot  but 
admire  the  rare  talent  for  observation  possessed  by  Wolff, 
who  actually  proved  the  most  important  facts  known  in 
this  the  darkest  portion  of  Embryology.  From  this  very 
difficult  investigation  he  even  drew  the  correct  conclusion 
that  the  entire  embryonic  body  of  all  higher  animals,  as 
well  as  of  birds,  is  for  a  while  a  flat,  thin,  leaf-shaped 
plate,  which  at  first  appears  simple,  but  subsequently 
as  if  composed  of  several  layers.  The  lowest  of  all  these 


WOLFF   ON   GERM-LAYERS.  45 

layers,  or  leaves,  is  the  intestinal  canal,  the  development  of 
which  Wolff  examined  thoroughly,  from  its  beginning  to  its 
completion.  He  showed  that  the  leaf-like  rudiment  first 
forms  a  groove,  the  edges  of  which  curve  towards  each  other, 
thus  growing  into  a  closed  tube,  and  that,  finally,  at  the 
ends  of  this  tube  the  two  openings,  mouth  and  anus,  arise. 

Nor  did  Wolff  overlook  the  fact  that  the  other  organic 
systems  of  the  body  originate,  in  an  entirely  similar  way 
from  leaf-shaped  rudiments,  which  afterwards  assume  the 
form  of  tubes.  Like  the  intestinal  canal,  the  nerve,  muscle, 
and  vascular  systems,  with  all  the  various  organs  belonging 
to  the  last,  develop  from  a  simple  layer-like  or  leaf-like 
rudiment.  Thus  in  1768  Wolff  learned  the  very  significant 
fact,  which,  half  a  century  later,  was  first  formulated  by 
Pander,  in  the  fundamental  "germ-layer  theory."  The 
sentence  in  which  Wolff  expressed  the  main  idea  of  this 
theory  is  so  remarkable,  that  I  quote  it.  "This  very 
wonderful  analogy  between  parts  which  in  Nature  are  so 
widely  separated,  an  analogy  which  is  not  imaginary,  but 
is  founded  on  the  most  reliable  observations,  is  in  the 
highest  degree  worthy  of  the  attention  of  physiologists ; 
for  it  will  be  granted  that  it  has  a  deep  significance  and 
that  it  is  most  intimately  connected  with  the  generation, 
and  with  the  nature  of  animals.  The  different  systems 
which  compose  the  whole  animal  seem  to  be  successively 
formed,  at  different  times  but  on  one  plan;  and  these 
systems  are  therefore  like  one  another,  even  though  in  their 
nature  they  are  distinct.  The  system  which  is  first  pro- 
duced, which  first  assumes  a  peculiar  definite  form,  is  the 
nerve-system.  When  this  is  completed  the  flesh-mass, 
which  properly  speaking  constitutes  the  embryo,  is  formed 


4.6  THE  EVOLUTION   OF  MAN. 

on  the  same  plan.  A  third,  the  vascular  system,  now  appears, 
which  is  certainly  sufficiently  similar  to  the  two  earlier 
structures  to  allow  of  its  form  being  easily  recognized  as 
that  which  has  been  described  as  approximately  common  to 
all  the  systems.  The  fourth  system,  the  intestinal  canal,  now 
follows ;  this,  again,  is  formed  on  the  same  plan,  and,  when 
completed  and  closed,  resembles  the  three  earlier  systems." 
In  this  most  important  discovery  Wolff  laid  the  first 
foundations  of  the  fundamental  "  germ-layer  theory  "  which 
was  not  completely  developed  till  long  afterwards,  by 
Pander  (1817)  and  by  Baer  (1828).  It  is  true  that  Wolff's 
propositions  are  verbally  incorrect,  but  in  them  he  reached 
the  truth  as  nearly  as  was  then  possible,  and  as  was  to  be 
expected.  We  shall  presently  see  how  nearly  he  approached 
to  the  real  state  of  the  case. 

Wolff  owes  much  of  his  comprehensive  conception  of 
nature  to  the  fact,  that  he  was  as  good  a  botanist  as  a 
zoologist.  He  studied  the  history  of  the  development  of 
plants  also,  and  in  the  field  of  botany  first  founded  the 
theory  which  Goethe  afterwards  developed  in  his  brilliant 
treatise  on  the  "  Metamorphosis  of  Plants."  Wolff  was  the 
first  to  show  that  all  the  various  parts  of  plants  may  be 
traced  back  to  the  leaf  as  their  common  rudiment,  or 
'  fundamental  organ."  Flower  and  fruit,  with  all  their 
parts,  consist  only  of  modified  leaves.  This  discovery  must 
have  seemed  all  the  more  surprising  to  Wolff,  from  the  fact 
that  he  had  discovered  a  simple  leaf-like  rudiment  to  be 
the  first  form  of  the  embryonic  body  of  animals,  as  it  is  of 
planta 

We  therefore  find  in  Wolff  distinct  traces  of  those 
theories  of  which,  at  a  much  later  period,  other  gifted 


WOLFF  AS  A   MONISTIC   PHILOSOPHER.  47 

naturalists  were  to  construct  the  foundation  of  the  know- 
ledge of  the  morphology  of  the  animal  and  vegetable  body. 
But  our  admiration  for  this  eminent  genius  is  still  greater 
when  we  discover  that  he  also  first  indicated  the  famous 
cellular  theory.  Indeed,  Wolff  had,  as  Huxley  first  pointed 
out,  an  evident  presentiment  of  this  fundamental  theory, 
for  he  considered  minute  microscopical  vesicles  to  be  the 
real  elementary  parts  constituting  the  germ-layers. 

Finally,  particular  attention  must  be  directed  to  the 
monistic  character  of  the  profound  philosophical  reflections 
which  Wolff  published  in  connection  with  all  his  admirable 
investigations.  Wolff  was  a  great  monistic  Natural  Phi- 
losopher, in  the  best  and  most  correct  sense  of  the  word. 
It  is  true  that  his  philosophical  researches,  like  his  ex- 
perimental ones,  were  ignored  for  more  than  half  a  century, 
and  have  not  even  yet  met  with  the  recognition  which 
they  deserve ;  but  we  therefore  emphasize  yet  more 
strongly  the  fact  that  their  tendency  was  strictly  in  that 
line  of  philosophy  which  we  call  monistic,  and  which  alone 
Ciui  be  considered  correct 


CHAPTER  III. 
MODERN  ONTOGENY. 

KA.RL  EIINST  BAEB. 

Karl  Ernst  Baer,  the  Principal  Disciple  of  Wolff.— The  Wiirzbnrg  School  of 
Embryologists :  Dollinger,  Pander,  Baer. — Pander's  Theory  of  Germ- 
layers. — Its  Full  Development  by  Baer. — The  Disc-shaped  first  parts 
into  two  Germ-layers,  each  of  which  again  divides  into  Two  Strata. 
The  Skin  or  Flesh-stratum  arises  from  the  Outer  or  Animal  Germ-layer. 
The  Vascular  or  Mucous  Stratum  arises  from  the  Inner  or  Vegetative 
Germ-layer.  The  Significance  of  the  Germ-layers. — The  Modification 
of  the  Layers  into  Tubes.  — Baer's  Discovery  of  the  Human  Egg,  the 
Germ-vesicle,  and  Chorda  Dorsalis. — The  Four  Types  of  Evolution  in 
the  Four  Main  Groups  of  the  Animal  Kingdom. — Baer's  Law  of  the  Type 
of  Evolution  and  the  Degree  of  Perfection. — Explanation  of  this  Law  by 
the  Theory  of  Selection. — Baer's  Successors  :  Rathke,  Johannes  Miiller, 
Bischoff,  Kolliker.— The  Cell  Theory :  Schleiden,  Schwann.— Its  Appli- 
cation to  Ontogeny  :  Robert  Remak. — Retrogressions  in  Ontogeny  : 
Eeichert  and  His. — Extension  of  the  Domain  of  Ontogeny  :  Darwin. 

"  The  History  of  Evolution  is  the  real  source  of  light  in  the  investigation 
of  organic  bodies.  It  is  applicable  at  every  step,  and  all  our  ideas  of  th« 
correlation  of  organic  bodies  will  be  swayed  by  our  knowledge  of  the 
history  of  evolution.  To  carry  the  proof  of  it  into  all  branches  of  research 
would  be  an  almost  endless  task." — KARL  ERNST  BAER  (1828). 

IF  we  wish  to  separate  our  historic  survey  of  the  course 
of  the  development  of  the  Science  of  Human  Ontogeny 
into  parts,  it  is  most  convenient  to  make  three.  The  first 
of  these  occupied  the  last  chapter,  and  includes  the  whole 
preparatory  period  of  embryologies!  researches;  it  extends 


THREE   PERIODS   IN   THE   HISTORY   OF   ONTOGENY.  49 

from  Aristotle  to  Caspar  Friedricli  Wolff,  to  the  year  1759, 
when  the  Theoria  Generationis  appeared  and  laid  the 
foundation  for  future  work.  The  second,  to  which  we  now 
turn  our  attention,  comprises  exactly  a  century ;  that  is, 
to  the  year  1859,  in  which  appeared  Darwin's  work  on 
"  The  Origin  of  Species,"  which  reformed  the  whole  basis  of 
the  science  of  Biology,  and  especially  of  Ontogeny.  The 
beginning  of  the  third  division  is  as  recent  as  the  time  of 
Darwin. 

As  Wolff's  labours  remained  entirely  unnoticed  during 
half  a  century — till  the  year  1812 — we  are  not  quite 
accurate  in  assigning  the  exact  duration  of  a  century  to  the 
second  division.  During  fifty-three  years  not  one  book 
appeared  which  followed  in  the  lines  laid  down  by  Wolff, 
and  carried  on  his  Theory  of  Evolution,  His  opinions, 
which  were  perfectly  correct  and  founded  directly  on  actual 
observations,  were  only  occasionally  mentioned,  and  then 
only  to  be  rejected  as  erroneous.  His  opponents,  followers 
of  the  prevalent  and  mistaken  theory  of  Pre -formation,  did 
not  even  deign  to  refute  him.  This  was  owing,  as  I  have 
said  before,  to  the  extraordinary  authority  possessed  by 
Albrecht  Haller,  Wolff's  distinguished  opponent,  and  the 
circumstance  furnishes  one  of  the  most  remarkable  examples 
of  the  influence  which  a  great  authority  may,  as  such,  long 
exert  against  the  clear  recognition  of  facts.  The  neglect 
of  Wolff's  labours  was  so  universal  that  in  the  beginning 
of  this  century  two  naturalists,  Oken  (1806)  and  Kieser 
(1810),  undertook  independent  investigations  into  the 
development  of  the  intestinal  canal  in  the  Chick,  and 
obtained  a  correct  insight  into  Ontogeny,  without  being 
aware  of  the  existence  of  Wolffs  important  work  in  the 


£0  THE   EVOLUTION    OF   MAN. 

same  field,  and  trod  in  his  very  footsteps  unconsciously. 
That  they  really  did  not  know  his  works  is  proved  by  the 
fact  that  they  did  not  advance  as  far  as  Wolff  had  done. 
Tn  the  year  1812  when  Meckel  translated  Wolff's  book  on 
the  Evolution  of  the  Intestinal  Canal  into  German,  and 
called  attention  to  its  great  importance,  the  eyes  of  anato- 
mists and  physiologists  were  for  the  first  time  suddenly 
opened,  and  a  great  number  of  Biologists  soon  after  under- 
took new  embryological  investigations,  following  out  and 
corroborating  Wolff's  theory  step  by  step. 

This  revival  of  Ontogeny,  and  the  first  confirmation  and 
further  development  of  the  only  true  theory  of  Epigenesis, 
started  from  the  university  of  Wiirzburg.  The  distinguished 
biologist,  Db'llinger,  was  then  lecturing  there.  He  was  the 
father  of  the  famous  theologian  of  Munich,  who  has  done 
such  good  service  in  our  day  by  his  opposition  to  the  new 
dogma  of  papal  infallibility.  Dollinger  was  both  a  thought- 
ful natural  philosopher,  and  an  accurate  biological  observer. 
He  felt  the  greatest  interest  in  the  History  of  Evolution, 
and  was  much  occupied  with  it.  Yet  he  himself  was  unable 
to  produce  any  very  important  work  in  this  department, 
from  want  of  means.  But  in  the  year  1816,  a  young  doctor 
of  medicine,  who  had  just  graduated,  and  whom  we  shall 
soon  learn  to  know  as  the  most  important  follower  of  Wolff, 
came  to  Wiirzburg.  This  was  Karl  Ernst  Baer.  His  con- 
versations with  Dollinger  on  the  History  of  Evolution 
resulted  in  a  renewal  of  the  investigations.  Dollinger  ex- 
pressed a  wish  that,  under  his  direction,  some  young 
naturalist  should  undertake  a  series  of  independent  re- 
searches into  the  evolution  of  the  Chick  during  the  hatching 
of  the  egg.  But  neither  he  nor  Baer  possessed  the  con> 


DOLLINGER,  BAER,  AND  PANDER.  51 

siderable  pecuniary  means  then  necessary  to  provide  a 
hatching-apparatus,  such  as  would  afford  uninterrupted 
observations  of  the  process,  or  to  pay  a  skilled  artist  t\> 
depict  in  a  reliable  form  the  successive  stages  of  develop- 
ment. They,  therefore,  confided  the  execution  of  the  plar. 
to  Christian  Pander,  a  wealthy,  early  friend  of  Baer's,  by 
whom  he  had  been  induced  to  come  to  Wurzburg.  Dalton, 
a  skilful  artist,  was  engaged  to  prepare  the  necessary  copper- 
plates. 

Thus  was  formed,  as  Baer  says,  "  that  combination,  ever 
memorable  in  the  history  of  science,  in  which  a  veteran,  grown 
gray  in  physiological  researches  (Dollinger),  a  youth  glowing 
with  zeal  for  science  (Pander),  and  an  artist  without  a  peer 
(Dalton),  united  their  powers  to  lay  a  firm  foundation  for 
the  History  of  the  Evolution  of  the  Animal  Organism."  In 
a  short  time  the  history  of  the  evolution  of  the  Chick,  in 
which  Baer  took,  though  Indirectly,  a  most  active  part, 
was  so  far  advanced  that  Pander,  in  his  dissertation 20  for 
the  degree  of  doctor,  published  in  1817,  was  able  to  give 
the  first  complete  sketch  of  the  history  of  the  evolution  of 
the  Chick  on  the  basis  of  Wolffs  theory.  He  was  able  to 
define  clearly  Wolff's  Theory  of  Germ-leaves,  and  to  prove 
from  observation  the  evolution  of  the  complex  system  of 
organs  from  simple  leaf-shaped  primitive  organs,  as  anti- 
cipated by  Wolff.  According  to  Pander,  the  leaf-shaped 
germinal  appendage  of  the  hen's  egg  separates  before  the 
twelfth  hour  of  incubation  into  two  distinct  layers — an 
outer  serous  layer,  and  an  inner  mucous  layer.  Between 
the  two,  a  third,  vascular  layer,  subsequently  developa 

Baer,  who  was  one  of  those  most  active  in  inducing 
Pander  to  make  his  investigations,  and  who  retained  the 


52  THE   EVOLUTION   OF   MAN. 

liveliest  interest  in  them  after  his  departure  from  Wurzburg, 
began  his  own  much  more  comprehensive  researches  in 
]  819,  and  nine  years  later  published,  as  the  fruit  of  these 
i  esearches,  a  work  on  "  The  History  of  the  Evolution  of 
Animals,"  which  even  now  is  generally  and  rightly  con- 
sidered the  most  important  and  valuable  contribution  to 
embryological  literature.  This  book,  a  true  model  of  careful, 
experimental  investigation,  combined  with  ingenious  philo- 
sophical speculation,  appeared  in  two  parts ;  the  first  in 
the  year  1828,  the  second  in  1837.21  It  is  the  firm  founda- 
tion on  which  the  whole  history  of  the  evolution  of  the 
individual  rests  to  this  day,  and  so  far  surpasses  its  pre- 
decessors, including  Pander's  outline,  that,  next  to  the 
labours  of  Wolff,  it  must  be  regarded  as  the  most  important 
basis  of  modern  Ontogeny.  As  Baer,  who  died  at  Dorpat  in 
November,  1876,  was  one  of  the  greatest  naturalists  of  our 
century,  and  has  exerted  a  most  important  influence  on 
other  branches  of  Biology  also,  it  may  be  of  interest  to  give 
some  account  of  the  life  of  this  extraordinary  man. 

Karl  Ernst  Baer  was  born  in  1792,  in  Esthonia,  on  the 
little  estate  of  Piep,  which  his  father  owned.  He  studied 
at  Dorpat  from  1810  to  1814,  and  then  went  to  Wurzburg, 
where  Dollinger  not  only  initiated  him  into  Comparative 
Anatomy  and  Ontogeny,  but  also  exercised  over  him,  by 
his  own  interest  in  philosophical  studies,  a  highly  stimu- 
lating influence.  From  Wiirzburg  Baer  went  to  Berlin, 
and  then,  accepting  a  call  from  the  physiologist  Burdach, 
to  Konigsberg.  There  he  delivered  lectures  on  Zoology  and 
Evolution,  with  some  interruptions,  until  1834,  and  com- 
pleted his  most  important  works.  In  1834  he  went  to  St 
Petersburg  as  a  member  of  the  Academy  of  that  place 


BAERS   WORK.  53 

There,  however,  he  forsook  almost  entirely  his  former  field 
of  labour,  and  occupied  himself  with  researches  of  a  totally 
different  nature,  in  various  branches  of  Natural  Science, 
especially  in  Geography,  Geology,  Ethnography,  and  Anthro- 
pology. His  works  on  the  History  of  the  Evolution  of 
Animals  are  far  the  most  important;  nearly  all  of  these 
were  completed  while  he  was  in  Kbnigsberg,  though  some 
of  them  were  not  published  until  later.  Their  merits,  like 
those  of  Wolffs  writings,  are  many-sided,  and  extend  over 
the  whole  domain  of  Ontogeny  in  very  various  directions. 

Baer  especially  perfected  the  fundamental  Theory  of 
Germ-layers,  as  a  whole  as  well  as  in  detail,  so  clearly  and 
completely,  that  his  idea  of  it  yet  forms  the  safest  basis  of 
our  knowledge  of  Ontogeny.  He  showed  that  in  Man  and 
the  other  Mammals,  as  in  the  Chick — in  short,  as  in  all  Ver- 
tebrates— first  two,  and  then  four  germ-layers  are  formed, 
always  in  the  same  manner,  and  that  the  modification  of 
these  into  tubes  gives  rise  to  the  first  fundamental  organs 
of  the  body.  According  to  Baer,  the  first  rudiment  of  the 
body  of  a  Vertebrate,  as  it  appears  on  the  globular  yelk 
of  the  fertilized  egg,  is  an  oblong  disc,  which  firtst  separates 
into  two  leaves  or  layers.  From  the  upper  or  animal  layer 
evolve  all  the  organs  which  produce  the  phenomena  of 
animal  life  :  the  functions  of  sensation,  of  motion,  and  the 
covering  of  the  body.  From  the  lower  or  vegetative  layer 
proceed  all  the  organs  which  bring  about  the  growth  of  the 
body :  the  vital  functions  of  nutrition,  digestion,  blood- 
making,  breathing,  secretion,  reproduction,  and  the  like. 
Each  of  these  two  original  germ-layers  separates  again  into 
two  thinner  layers,  or  lamellae,  one  lying  above  the  other. 
First,  the  animal  layer  separates  into  two,  which  Baer  calls 


54  THE   EVOLUTION   OF  MAN. 

the  skin,  or  dermal  layer,  and  the  flesh,  or  muscular  layei. 
From  the  uppermost  of  these  two  lamellae,  the  skin-layer, 
are  formed  the  outer  skin,  the  covering  of  the  body,  and  the 
central  nervous  system,  the  spinal  cord,  the  brain,  and 
the  organs  of  sensation.  From  the  lower,  or  flesh-layer, 
the  muscles,  or  fleshy  parts,  the  internal  or  bony  skeleton, — 
in  short,  the  organs  of  motion,  arise.  Secondly,  the  lower, 
or  vegetative  germ-layer,  parts  in  the  same  way  into  two 
lamellae,  which  Eaer  distinguishes  as  the  vascular  and  the 
mucous  layer.  From  the  outer  of  the  two,  the  vascular 
layer,  proceed  the  heart  and  the  blood-vessels,  the  spleen, 
and  the  other  so-called  blood-vessel  glands,  the  kidneys, 
and  the  sexual  glands.  Finally,  from  the  lowest,  and  fourth 
or  mucous  layer,  arises  the  inner  alimentary  membrane  of 
the  intestinal  canal,  with  all  its  appendages,  liver,  lungs, 
salivary  glands.  Baer  traced  the  transformation  of  these 
four  secondary  germ-layers  into  tube-shaped  fundamental 
organs  as  ingeniously  as  he  had  successfully  determined 
their  import  and  their  formation  in  pairs  by  the  segmen- 
tation of  the  two  primary  germ-layers.  He  was  the  first 
to  solve  the  difficult  problem  as  to  the  process  by  which 
the  entirely  different  body  of  the  vertebrate  develops  from 
this  flat,  leaf-shaped,  four-layered  original  germ  ;  the  process 
was  the  transformation  of  the  layers  into  tubes. 

In  accordance  with  certain  laws  of  growth,  the  flat 
layers  bend,  and  become  arched ;  the  edges  grow  towards 
each  other  so  that  the  distance  between  them  is  continually 
decreased ;  finally  they  unite  at  the  point  of  contact.  By 
this  process  the  flat  intestinal  layer  changes  into  a  hollow 
intestinal  tube ;  the  flat  spinal  layer  becomes  a  hol]o\v 
spinal  tube,  the  skin-layer  becomes  a  skin-tube,  etc. 


DISCOVERY   OF   THE   HUMAN   EGG.  55 

Among  the  many  and  great  services  which  Baer  ren- 
dered in  detail  to  Ontogeny,  especially  to  that  of  Vertebrates, 
his  discovery  of  the  human  egg  must  be  especially  men- 
tioned here.  Most,  even  of  the  earlier  naturalists,  had 
assumed  that  man  proceeds,  like  other  animals,  from  an 
egg.  The  Theory  of  Evolution  (pre-formation)  had,  more- 
over, assumed  that  all  past,  present,  and  future  generations 
of  the  human  race  existed  encased  in  the  ova  of  Eve,  the 
common  mother.  Yet  the  ova  of  Man  and  other  Mammals 
were  not  actually  known  till  the  year  1827.  For  the  egg 
is  exceedingly  small,  a  spherical  vesicle  or  bladder  of  only 
one-tenth  of  a  line  in  diameter,  which  can  be  seen  with  the 
naked  eye  only  under  very  favourable  circumstances.  This 
spherical  vesicle,  when  in  the  ovary  of  the  mother,  is  en- 
closed in  a  number  of  peculiar  spherical  vesicles  of  much 
larger  size,  called  Graafian  follicles,  after  their  discoverer 
Graaf,  and  these  were  formerly  universally  regarded  as  the 
actual  eggs.  It  was  not  until  the  year  1827 — not  fifty  years 
ago — that  Baer  proved  that  these  Graafian  follicles  are  not 
the  actual  eggs,  which  are  much  smaller,  and  only  imbedded 
in  the  Graafian  follicles.  (Of.  end  of  Chapter  XXV.) 

Baer  was  also  the  first  to  observe  the  so-called  germinal 
vesicle  of  Mammals,  that  is,  the  little  spherical  bladder 
which  is  first  developed  from  the  impregnated  egg,  and  the 
(Lin  wall  of  which  consists  of  a  single  layer  of  uniform 
polygonal  cells.  (See  Chapter  VIII.)  Another  discovery  of 
Baer's,  of  great  importance  in  understanding  the  types  of 
the  lineage  of  the  Vertebrates,  and  the  characteristic 
organization  of  this  group  of  animals  in  which  Man  is 
included,  was  that  of  the  Chorda  Dorsalis.  This  is  a  long, 
thin,  cylindrical  cartilaginous  cord,  which  in  all  Vertebrates 


56  THE   EVOLUTION   OF   MAN. 

passes  lengthwise  through  the  whole  body  of  the  embryo. 
It  is  developed  at  a  very  early  stage,  and  is  the  first  form?,- 
tion  of  the  spine,  the  firm  axis  of  Vertebrates.  In  the 
Lancelet  (Amphioxus),  the  lowest  of  all  Vertebrates,  the 
entire  inner  skeleton  is  limited  to  this  Chorda  throughout 
life.  But  in  Man  and  all  the  higher  Vertebrates,  first  the 
spine,  and  later  the  skull,  are  developed  round  this  cord. 

Important  as  these  and  many  other  discoveries  of  Baer's 
were  in  the  Ontogeny  of  Vertebrates,  yet  the  great  im- 
portance of  his  researches  rested  especially  on  the  fact  that 
he  was  the  first  to  apply  the  comparative  method  to  the 
study  of  the  evolution.  It  was,  of  course,  the  Ontogeny  of 
Vertebrates,  and  principally  of  Birds  and  Fishes,  that  Baer 
first  and  especially  investigated.  Yet  he  by  no  means 
limited  himself  to  these ;  for  he  included  various  Inverte- 
brates in  his  investigations.  The  most  general  result  of 
these  comparative  embryological  researches  was  that  Baer 
assumed  four  totally  different  courses  of  evolution  for  the 
four  principal  groups  of  the  animal  kingdom.  These  four 
chief  groups,  or  types,  which  at  that  time  had  just  begun 
to  be  distinguished,  in  consequence  of  George  Cuvier's 
researches  in  Comparative  Anatomy,  are :  (1)  Vertebrates 
(Vertebrata) ;  (2)  Articulated  animals  (Arthropoda) ;  (3) 
Soft-bodied  animals  (Mollusca) ;  and  (4)  the  lower  animals, 
which  at  that  time  were  all  erroneously  grouped  under  the 
term  Radiata.  Cuvier,  in  the  year  1816,  demonstrated  for 
the  first  time  that  these  four  groups  of  the  animal  kingdom 
show  very  essential  and  typical  distinctions  in  their  whole 
inner  structure,  and  in  the  arrangement  and  position  of  the 
organic  systems;  that,  on  the  other  hand,  the  internal 
structure  of  all  animals  of  one  type,  for  example,  of  all  Ver- 


FOUR   TYFES   OF   DEVELOPMENT.  57 

fcebrates,  is  essentially  similar,  notwithstanding  the  great 
variety  of  outward  forms.  Baer,  however,  independently 
and  almost  simultaneously,  furnished  proof  that  the  four 
groups  develop  from  the  egg  by  entirely  different  processes, 
and  further,  that  the  order  of  the  series  of  embryonic  forms 
in  the  course  of  evolution  is  from  the  very  beginning 
identical  in  all  animals  of  the  same  type,  but,  on  the  other 
hand,  different-  in  those  of  different  types.  Up  to  that 
time,  in  making  a  classification  of  the  animal  kingdom,  an 
endeavour  had  always  been  made  to  arrange  all  animals,  from 
the  lowest  to  the  highest,  from  the  infusoria  to  man,  in 
a  single  connected  series  of  forms;  and  the  false  idea  had 
always  been  maintained,  that  there  was  a  single  unbroken 
gradation  of  development  from  the  lowest  animal  to  the 
highest.  Cuvier  and  Baer  proved  that  this  conception  is 
totally  erroneous, — and  that,  on  the  contrary,  there  are  four 
wholly  distinct  types  of  animals,  which  must  be  distin- 
guished not  only  as  to  their  anatomical  structure,  but  also 
as  to  their  embryonic  evolution. 

As  a  result  of  this  discovery,  Baer  succeeded  in  estab- 
lishing a  very  important  law,  which  we  shall  name  in  his 
honour  Baer's  Law,  and  which  he  expresses  as  follows  : 
"  The  evolution  of  an  individual  of  a  certain  animal  form 
is  determined  by  two  conditions :  firstly,  by  a  continuous 
perfection  of  the  animal  body  by  means  of  an  increasing 
histological  and  morphological  differentiation,  or  an  increas- 
ing number  and  diversity  of  tissues  and  organic  forms; 
secondly,  and  at  the  same  time,  by  the  continual  transition 
from  a  more  general  form  of  the  type  to  one  more  specific." 
The  degree  of  perfection  of  the  animal  body  depends  on 
the  greater  or  less  amount  of  heterogeneity  there  is  in  its 


58  THE   EVOLUTION   OF   MAN. 

elementary  parts,  and  in  the  segments  of  its  composite 
organs, — in  a  word,  in  the  degree  of  histological  and  mor- 
phological differentiation.  The  type,  on  the  other  hand, 
is  the  order  of  the  arrangement  of  the  organic  elements  and 
of  the  organs.  The  type  is  quite  distinct  from  the  degree  of 
perfection;  the  same  type  may  exist  in  several  degrees 
of  perfection ;  and,  conversely,  the  same  grade  of  perfection 
may  be  reached  in  several  types.  This  explains  the  phe- 
nomenon that  the  most  perfect  animals  of  any  type, — for 
example,  the  highest  Arthropods  and  Molluscs, — are  much 
more  perfectly  organized,  or  more  highly  differentiated, 
than  the  most  imperfect  animals  of  other  types, — for  ex- 
ample, than  the  lowest  Vertebrates  and  Star-animals. 

Baer's  Law  has  been  of  the  greatest  importance  in 
advancing  our  knowledge  of  animal  organization  ;  though 
it  was  not  until  a  later  period  that  Darwin  enabled  us  to 
perceive  and  value  its  real  significance.  Here  we  may 
at  once  remark  that  it  can  only  be  really  understood  by 
means  of  the  Theory  of  Descent,  by  the  recognition  of  the 
very  important  part  played  by  Heredity  and  Adaptation 
in  the  construction  of  organic  form.  As  I  have  shown  in 
my  Generclle  Morphologie  (vol.  ii.  p.  10),  the  type  of 
evolution  is  the  mechanical  result  of  Heredity;  the  degree 
of  perfection  is  the  mechanical  result  of  Adaptation. 
Heredity  and  Adaptation  are  the  mechanical  factors  in  the 
production  of  organic  forms,  which  were  first  brought  to 
bear  on  Ontogeny  by  Darwin's  Theory  of  Selection,  and 
which  have  enabled  us  for  the  first  time  to  understand 
Baer's  Law. 

Baer's  labours  marked  the  beginning  of  a  new  epoch, 
and  aroused  an  extraordinary  interest  in  embryological 


FRESH  IMPETUS  GIVEN  TO  ONTOGENY.         59 

research  throughout  a  very  wide  circle.  We  find,  therefore, 
that  a  large  number  of  investigators  occupied  the  newly 
found  field  of  research,  and,  with  praiseworthy  industry, 
made  a  great  number  of  distinct  new  facts  in  a  short  time. 
The  majority  of  these  new  embryologists  are  industrious 
specialists,  who  have  been  very  useful  in  collecting  fresh 
materials,  but  who  have,  as  a  rule,  done  but  little  to  ad- 
vance the  general  problem  of  the  History  of  Germs.  I  can, 
therefore,  limit  myself  to  the  mention  of  a  few  names. 
Of  special  importance  are  the  investigations  of  Heinrich 
Rathke,  of  Konigsberg  (died  1861),  who  did  much  to  advance 
the  History  of  the  Evolution  of  Invertebrates  (Crabs,  In- 
sects, Molluscs),  as  well  as  of  Vertebrates  (Fishes,  Turtles, 
Snakes,  Crocodiles).  In  the  subject  of  the  Embryology  of 
Mammals,  the  widest  conclusions  are  due  to  the  careful 
experiments  of  Wilhelm  Bischoff,  of  Munich.  His  History 
of  the  Evolution  of  the  Rabbit  (1840),  of  the  Dog  (1842),  of 
the  Guinea-Pig  (1852),  and  of  the  Roe-Deer  (1854),  are  as 
yet  the  most  important  basis  of  study  in  this  department. 
Among  the  numerous  works  on  the  History  of  the  Evolution 
of  Invertebrates,  those  of  the  well-known  zoologist,  Johannes 
Miiller,  of  Berlin,  on  Star-animals  (Echinoderma),  are  espe- 
cially noteworthy ;  also  those  of  Albert  Kolliker,  of  Wiirz- 
burg,  on  Cuttle-fishes  (Cephalopoda);  of  Siebold  and  Huxley, 
on  Worms  and  Plant-animals ;  of  Fritz  Miiller  (Desterro),  on 
the  Crustacea ;  of  Weismann,  on  Insects,  etc.  The  number  of 
labourers  in  this  field  has  of  late  greatly  increased,  although 
not  very  much  of  special  importance  has  been  accomplished. 
It  is  evident,  from  the  majority  of  recent  publications  on 
Ontogeny,  that  their  authors  are  not  familiar  enough  with 
Comparative  Anatomy.  The  most  important  of  the  latest 


6O  THE   EVOLUTION   OF  MAN. 

ontogenetic  works  are  those  of  Kowalevsky,  E.  Ray  Lan- 
kester,  and  Eduard  van  Beneden,  to  which  we  shall  presently 
again  refer.22 

A  more  decided  advance  in  general  knowledge  than  waa 
effected  by  all  these  separate  investigations,  dates  from  the 
year  1838,  when  the  proof  of  the  Cellular  Theory  suddenly 
opened  a  new  field  of  research  in  the  History  of  Evolution. 
The  distinguished  botanist,  Schleiden,  of  Jena,  having 
proved  by  means  of  the  microscope  that -every  vegetable 
body  is  composed  of  innumerable  elementary  parts,  the  so- 
called  cells,  Theodor  Sch  wann,  of  Berlin,  a  pupil  of  Johannes 
Miiller,  applied  this  discovery  directly  to  the  animal  body.23 
He  showed,  that  not  only  in  plants,  but  also  in  the  bodies 
of  the  most  dissimilar  animals,  these  same  cells  are  dis- 
tinguishable, under  the  microscope,  in  all  the  tissues,  and 
that  they  form  the  actual  building  material  of  organisms. 
.All  the  numerous  tissues  of  the  animal  body,  such  as  the 
entirely  dissimilar  tissues  of  the  nerves,  muscles,  bones, 
outer  skin,  mucous  skin,  and  of  other  similar  parts,  are 
originally  composed  of  cells;  and  the  same  is  true  of 
all  the  various  tissues  of  the  vegetable  body.  These  cells, 
which  we  shall  hereafter  consider  more  closely,  are  inde- 
pendent living  beings,  the  citizens  of  the  state,  which  con- 
stitute the  entire  multi-cellular  organism.  The  knowledge 
of  this  most  important  fact  was,  of  course,  of  direct  service 
to  the  History  of  Evolution  also,  in  that  it  raised  many 
new  questions,  chiefly  the.se :  What  relation  have  the  cells 
to  the  germ-layers  ?  Are  the  germ -layers  already  com- 
posed of  cells,  and  how  are  they  related  to  the  cells  of  the 
tissues  which  afterwards  appear  ?  What  place  does  tlie 
egg  hold  in  the  Cell  Theory  ?  Is  it  itself  a  cell,  or  is  it 


THE   CELL   THEORY.  6l 

composed  of  cells  ?  These  were  the  important  questions 
which  the  Cell  Theory  at  once  raised  in  the  study  of  Em- 
bryology. 

Several  naturalists  attempted  in  different  ways  to 
furnish  the  right  answers,  but  the  excellent  "  Investigations 
into  the  Evolution  of  Vertebrates,"  by  Robert  Remak,  of 
Berlin  (1851),  became  conclusive.  By  somewhat  remoulding 
the  Cellular  Theory  of  Schleiden  and  Schwann,  this  gifted 
naturalist  was  able  to  overcome  the  great  obstacles  which 
this  theory,  in  its  first  form,  had  placed  in  the  way  of 
Embryology.  It  is  true  that  the  anatomist,  Karl  Boguslaus 
Reichert,  of  Berlin,  had  previously  attempted  to  explain  the 
origin  of  the  tissues.  But  this  attempt  was  necessarily  a 
total  failure,  owing  to  the  fact  that  the  extraordinarily 
confused  mind  of  the  author  was  equally  destitute  of  every 
correct  idea  of  the  History  of  Evolution,  of  the  Cellular 
Theory  as  a  whole,  and  of  a  sound  view  of  the  structure 
and  development  of  tissues  in  particular.  The  inaccuracy 
of  Reichert's  observations,  and  the  falsity  of  the  conclusions 
drawn  from  them,  is  shown  by  every  accurate  test  applied 
to  his  so-called  discoveries.  By  way  of  illustration,  it  may 
be  said  that  he  declared  the  whole  of  the  upper  germ-layer, 
from  which  the  most  important  parts  of  the  body — brain, 
spinal  cord,  outer  skin,  and  the  like — proceed,  to  be  merely 
a  transient  "  enveloping-skin  "  of  the  embryo,  and  that  it 
had  nothing  to  do  with  the  formation  of  the  body ;  that 
many  of  the  first  formations  of  the  separate  organs  did  not 
proceed  from  the  primary  germ-layers,  but  came  one  by 
one  from  the  yelk  of  the  egg,  and  joined  the  layers  after- 
ward. Reichert's  preposterous  embryological  labours  suc- 
ceeded in  gaining  a  passing  attention,  only  because  they 


62  THE   EVOLUTION   OF   MAN. 

were  put  forward  with  unusual  presumption,  and  professed 
to  disprove  Baer's  Theory  of  Germ-layers.  They  are 
written  in  so  clumsy  and  confused  a  style,  that  no  one 
could  quite  understand  them  ;  but  for  this  very  reason  they 
won  the  admiration  of  many  readers,  who  supposed  that 
a  nucleus  of  profound  wisdom  was  hidden  somewhere  be- 
hind these  obscure  oracular  and  mysterious  sayings. 

Remak  was  the  first  to  throw  full  light  on  the  great 
conftbdon  which  Reichert  had  caused,  by  explaining,  in  the 
simplest  possible  manner,  the  evolution  of  the  tissues.  Ac- 
cording to  him,  the  egg  of  animals  is  always  a  simple 
cell,  and  the  germ-layers,  which  proceed  from  the  egg,  are 
also  composed  only  of  cells,  and  those  cells,  which  alone 
constitute  the  egg,  are  produced  in  a  very  simple  manner 
by  the  continuous  and  repeated  segmentation  or  dividing 
up  of  the  original  simple  egg-cell.  This  cell  divides,  or 
parts,  first  into  two,  and  then  into  four;  from  these  four 
arise  eight,  then  sixteen,  and  then  thirty-two,  and  so  on. 
Hence,  in  the  individual  evolution  of  every  animal,  as  well 
as  of  every  plant,  from  the  one  simple  cell,  constituting  the 
egg,  is  formed,  by  repeated  segmentation,  an  aggregate  of 
cells,  as  Kolliker  had  already  maintained  in  1844.  The 
cells  of  such  a  mass  spread  themselves  out  flatly,  and  so 
form  into  layers,  so  that  every  one  of  these  layers  is 
originally  composed  of  but  one  kind  of  cell.  The  cells  of 
the  layers  differentiate  themselves,  or  assume  various  forms ; 
and  then  there  is  a  further  differentiation,  or,  in  other 
words,  a  division  of  labour  of  the  cells  within  the  layers 
themselves,  and  this  latter  differentiation  produces  all  the 
various  tissues  of  the  body. 

These  are  the  very  simple  principles  of  Histogeny,  01 


REMAK.  63 

the  Science  of  the  Evolution  of  Tissues,  as  first  elaborated 
by  Remak  and  by  Kblliker  in  this  comprehensive  sense. 
By  thus  proving  more  definitely  the  part  which  the  germ- 
layers  take  in  the  formation  of  the  various  tissues  and 
systems  of  organs,  and  applying  the  Theory  of  Epigenesis 
to  the  cells  and  the  tissues  formed  from  them,  Remak  raised 
the  Germ-layer  Theory,  at  least  as  regards  Vertebrates,  to 
that  degree  of  perfection  in  which  we  shall  find  it  hereafter 
when  we  examine  it  in  detail.  According  to  him,  the  two 
germ-layers,  of  which  the  so-called  germinal  disc,  the  first 
simple  leaf-shaped  formation  of  the  body  of  a  Vertebrate, 
is  composed,  are  soon  increased  by  another  layer,  produced 
by  the  lower  layer  separating  into  two.  These  three  have, 
entirely  distinct  relations  to  the  various  tissues.  First, 
from  the  upper  layer  proceed  those  cells  which  compose  the 
outer  skin  (epidermis)  of  the  body,  together  with  the  parts 
belonging  and  necessary  to  it  (hair,  nails,  and  the  like) — 
that  is,  the  external  covering  which  envelops  the  whole 
body ;  and,  remarkable  as  it  is,  it  produces  also  the  cells 
which  constitute  the  central  nervous  system, — the  brain  and 
spinal  marrow.  Secondly,  from  the  lower  germ-layer  spring 
the  cells  which  form  the  intestinal  epithelium, — that  is  the 
whole  inner  coating  of  the  intestinal  canal  and  its  append- 
ages (liver,  lungs,  salivary  glands,  and  the  like) ;  in  other 
words,  the  tissues  which  take  up  the  food  of  the  animal  body 
and  attend  to  its  digestion.  Finally,  from  the  middle  layer, 
lying  between  these  two,  arise  all  the  other  tissues  of  the 
body  of  the  Vertebrate;  flesh  and  blood,  bones  and  liga- 
ments, and  the  like.  Remak  also  proved  that  the  middle 
layer,  which  he  calls  the  "  motor-germinative  "  layer,  again 
separates,  secondarily,  into  two  layers.  In  this  way  we  get 


64  THE   EVOLUTION   OF  MAN. 

the  same  four  layers  which  Baer  had  previously  assumed. 
The  upper  part  of  the  middle  layer  after  its  cleavage 
(Baer's  "Flesh-stratum"),  Remak  calls  the  skin-lamella 
(Hautplatte,  or  better,  Hautfaserplatte);  it  forms  the  outer 
wall  of  the  body  (the  true  skin,  cutis  vena,  the  muscles, 
bones,  and  the  like).  The  lower  part  (Baer's  "Vascular 
stratum"),  he  calls  the  intestinal-fibrous  lamella  (Darm- 
faserplatte) ;  it  forms  the  outer  covering  of  the  intestinal 
canal,  and  of  the  heart,  the  blood-vessels,  and  so  on. 

Based  on  the  firm  foundation  which  Remak  thus  supplied 
to  the  History  of  the  Evolution  of  the  Tissues,  or  the  science 
called  Histogeny,  numerous  investigations  of  special  points 
which  have  considerably  extended  our  information  have 
been  made.  Of  course  many  attempts  have  been  made 
to  give  much  narrower  limitations  to  Remak's  doctrines,  or 
to  remodel  them  altogether.  Reichert,  of  Berlin,  and  Wil- 
helm  His,  of  Leipsic,  have  specially  busied  themselves  to 
establish,  in  comprehensive  works,  an  entirely  new  view  of 
the  evolution  of  the  body  of  Vertebrates,  according  to  which 
the  rudiments  of  the  body  of  the  Vertebrate  does  not  consist 
solely  of  the  two  primary  germ-layers.  But  these  works, 
owing  to  their  total  lack  of  the  necessary  knowledge  of 
Comparative  Anatomy,  and  clear  knowledge  of  Ontogeny, 
and  to  the  fact  that  they  do  not  even  glance  at  Phylogeny, 
could  exert  but  a  very  transient  influence.  Only  the  total 
want  of  critical  ability  and  comprehension  of  the  real 
problems  of  the  History  of  Evolution  can  explain  the  fact 
that  many  people  for  a  time  regarded  the  strange  fancies  of 
Reichert  and  His  as  a  great  gain. 

His,  in  1868,  in  a  large  book,  on  "  The  Eaily  Evolution 
of  the  Chick  in  the  Egg,"  detailed  his  entirely  erroneous 


HIS  AND   GOETTK.  6$ 

views  in  a  very  learned  form,  and  under  the  banner  of  a 
new  and  very  exact  mathematical  and  physical  method,  he 
has  recently  expressed  the  same  views  in  a  general  form  in 
his  book  on  "  Our  Body  and  the  Physiological  Problem  of 
its  Origin"  (Leipsic,  1875).  As  His,  in  order  to  increase 
the  circulation  of  the  latter  book,  has  allowed  it  to  be 
publicly  advertised  as  "  important  to  readers  of  Haeckel's 
Anthropogenic,"  I  shall  only  remark  that  my  treatise  on 
"  The  Aims  and  Methods  of  the  History  of  Evolution " 
(Jena,  1875)  frees  me  from  the  necessity  of  further  answer. 
To  the  most  important  points  in  his  false  theories  I  shall 
refer  again.  (See  Chapter  XXIV.) 

Quite  recently,  however,  His  and  Reichert's  books  on 
Ontogeny,  which  had  previously  ranked  as  the  most  per- 
verted and  unfortunate  of  the  larger  works  on  this  science, 
have  been  far  eclipsed,  in  that  respect,  by  a  ponderous  work 
by  Alexander  Goette,  of  Strasburg,  on  the  "  History  of  the 
Evolution  of  Bombinator  igneus,  as  the  Basis  of  a  Com- 
parative Morphology  of  Vertebrates  "  (Leipsic,  1875).  This 
monograph  is  the  biggest  existing  contribution  to  the 
literature  of  Ontogeny — a  thick  volume  of  964  pages,  ac- 
companied by  a  very  beautiful  folio  atlas  of  22  plates. 
These  splendid  plates,  containing  as  many  as  382  accurate 
and  very  carefully  executed  drawings,  representing  the 
history  of  the  development  of  the  Bombinator,  are  the 
result  of  years  of  incessant  labour,  and  excite  a  most 
favourable  interest  in  the  huge  work.  Unfortunately, 
however,  the  reader  who  is  induced  by  this  splendid 
picture-book  to  expect  a  corresponding  degree  of  excellence 
in  the  voluminous  text,  will  be  sadly  disappointed.  Not 
only  is  the  whole  account  most  obscure,  confused,  and 


66  THE   EVOLUTION   OF  MAN. 

contradictory,  but,  further,  the  entire  treatment  shows  that, 
by  his  whole  scientific  education,  the  author  is  incapable 
of  the  heavy  task.  I  should  not  pronounce  this  harsh 
judgment,  but  that  Goette  natters  himself  that,  as  the 
reformer  of  the  science,  he  is  about  to  place  it  on  an 
entirely  new  basis ;  and  but  that,  consequently,  he  treats 
the  great  leaders  of  the  science — Baer,  Remak,  Gegenbaur, 
and  others — in  the  most  insolent  manner,  as  narrow-minded 
labourers  who,  "  by  reason  of  their  lack  of  knowledge  of  the 
history  of  evolution,  have  missed  their  aim."  The  following 
samples  seem  to  show  the  mode  in  which  the  new  science 
is  constituted  by  Goette :  "  Perfect  life  renders  evolution 
impossible.  The  capacity  of  evolution  in  the  mature  egg 
excludes  real  life.  Egg-cleavage  is  not  a  living  process  of 
evolution.  The  egg  neither  as  a  whole  nor  as  to  its  parts, 
neither  in  its  origin  nor  in  its  complete  state,  is  a  cell.  The 
cells  of  the  various  tissues  are  not  organisms,  are  not  organic 
individuals.  The  individuality  of  an  organism  is  only  a 
peculiar  expression  of  the  end  of  its  evolution,"  and  so  on. 

In  these  and  many  other  statements  Goette  abruptly 
upsets  the  whole  science,  as  at  present  constituted.  The 
Cell  Theory  and  the  Protoplasmic  Theory  are  rejected  as 
worthless  ;  even  Comparative  Anatomy  is,  according  to  this 
writer,  of  no  scientific  value ;  Phylogeny  is  no  science,  and 
so  on.  I  have  explained  the  most  incredible  of  Goette's 
assertions  and  his  most  unexampled  errors  in  my  work  on 
"  The  Aims  and  Methods  of  the  History  of  Evolution " 
(Leipsic,  1875) ;  in  which  book  I  have  also  criticized  the 
views  held  by  His  and  Agassiz.  Errors  of  this  sort  are  no 
longer  possible  in  other  sciences.  Their  occurrence  in  the 
History  of  Evolution  is  explained  partly  by  the  great 


HUXLEY    AND    KOWALEVSKY.  67 

difficulty  of  the  very  complex  task  which  lies  before  this 
science,  and  partly  by  the  insufficient  general  preparation 
possessed  by  most  of  the  more  recent  students. 

All  valuable  modern  investigations  into  Animal  Onto- 
genesis have  only  tended  to  confirm  and  add  to  the  Theory 
of  Germ-layers  as  established  by  Baer  and  Remak.  As  the 
most  important  advance  made  in  this  direction,  it  is  deserv- 
ing of  mention,  that  the  same  two  primary  germ-layers, 
from  which  the  body  of  Vertebrates,  including  Man, 
develops,  have  recently  been  shown  to  exist  in  all  inver- 
tebrate animals  also,  with  the  single  exception  of  the  lowest 
group,  that  of  the  Primaeval  animals  (Protozoa.}  The  dis- 
tinguished English  naturalist,  Huxley,  in  the  year  1849. 
had  already  shown  that  this  is  also  true  of  Plant-animals 
(Meduscs).  He  drew  attention  to  the  fact  that  the  two 
cell-layers,  from  which  the  body  of  this  Plant-animal 
develops,  correspond,  morphologically  as  well  as  physio- 
logically, to  the  two  primary  germ -layers  of  Vertebrates. 
The  upper  germ-layer,  from  which  the  outer  skin  and  the 
flesh  proceed,  he  named  Ectoderm,  or  Outer  layer ;  the 
lower,  which  forms  the  organs  of  digestion  and  reproduc- 
tion, he  called  the  Entoderm,  or  Inner  layer.  But  during 
the  past  ten  years,  the  two  germ-layers  have  been  found  to 
exist  among  many  other  Invertebrates,  The  indefatigable 
Russian  zoologist,  Kowalevsky,  has  found  them  among 
widely  differing  groups  of  Invertebrates,  in  Worms, 
Star-animals  (Echinoderma),  Soft-bodied  animals  (Mollusca), 
Articulates  (Arthropoda),  and  the  like. 

In  my  Monograph  on  the  Calcareous  Sponges,  which 
appeared  in  1872, 1  have  shown  that  this  same  pair  of  primary 
germ-layers  forms  the  basis  of  the  body  of  the  Sponges,  and 


68  THE   EVOLUTION   OF  MAN. 

that  they  are  to  be  regarded  as  occupying  the  same  relative 
place,  or  as  being  homologous,  throughout  all  the  various 
classes  of  animals,  from  the  Sponges  to  Man.  This  homology 
of  the  two  primary  germ-layers,  which  is  of  extraordinary 
significance,  extends  throughout  the  animal  kingdom,  with 
only  a  few  exceptions  in  the  lowest  class,  the  Primaeval 
animals  (Protozoa).  These  animals  are  of  an  exceedingly 
low  organization,  and  do  not  advance  to  the  stage  of  form- 
ing germ-layers,  and  consequently  never  form  real  tissues. 
The  whole  body  merely  consists,  either  of  a  single  cell,  as 
in  Amrebae  and  Infusoria,  or  of  a  loose  mass  of  but  slightly 
differentiated  cells,  or,  as  in  Monera,  it  does  not  even  attain 
a  form  as  high  as  that  of  a  cell.  But  from  the  egg-cell  of 
all  other  animals  two  primitive  germ-layers  first  proceed, 
the  outer,  animal  layer  (Ectoderm  or  Exoderm),  and  the 
inner,  or  vegetative  layer  (Entoderm),  and  from  these  the 
various  tissues  and  organs  arise.  This  is  equally  true  of 
Sponges,  of  the  other  Plant-animals,  and  of  Worms ;  it  is  as 
true  of  Soft-bodied  animals  (Molluscd),  Star-animals  (Echin- 
oderma),  and  Articulates  (Arthropoda),  as  of  Vertebrates. 
All  these  animals  may  be  comprised  under  the  head  of 
Intestinal  Animals  (Metazoa),  in  distinction  from  the 
Primaeval  Animals  (Protozoa),  which  have  no  intestine. 

It  is  perhaps  more  correct  not  to  place  the  Protozoa 
among  the  true  animals  at  all,  but  to  class  them  in  the 
neutral  kingdom  of  the  Protista,  those  humblest  primaeval 
beings  which  are  neither  true  animals  nor  true  plants. 
According  to  this  view  the  Metazoa  can  alone  be  considered 
as  true  animals,  and  the  origin  from  two  primary  germ- 
layers  may  be  held  to  form  the  primary  character  of  tho 
animal  kingdom. 


DARWIN.  69 

In  the  lowest  forms  of  Metazoa,  the  body  consists 
throughout  life  of  these  two  primary  germ-layers.  But  in 
all  higher  Intestinal  Animals,  each  of  these  forms  by 
cleavage  two  other  layers,  so  that  the  body  is  thenceforward 
composed  of  four  secondary  germ-layers.  In  my  "  Gastraea 
Theory  "  (1873),  I  have  tried  to  show  the  general  homology 
of  these  four  layers  in  all  Metazoa,  and  I  have  pointed  out 
the  important  bearing  of  this  fact  on  the  natural  system  of 
the  animal  kingdom.24 

But  though  the  most  important  facts  in  the  individual 
evolution  of  the  human  and  animal  body  had  been  suffi- 
ciently established  by  these  advances  in  Animal  Ontogeny, 
yet  the  most  difficult  task  remained, — namely,  the  discovery 
of  'the  causes  by  which  the  evolution  of  organisms  and  the 
production  of  their  forms  is  effected.  The  real  mechanical 
causes  of  individual  evolution  were  first  explained  in  1859, 
in  Darwin's  work,  in  which  the  facts  of  Heredity  and 
Adaptation  were  for  the  first  time  scientifically  discussed, 
and  their  bearing  on  Ontogeny  correctly  interpreted.  Only 
by  the  Theory  of  Descent,  and  by  the  aid  of  the  laws  of 
Heredity  and  Adaptation,  are  we  really  able  to  understand 
the  facts  of  individual  evolution,  and  to  explain  them  by 
efficient  causes.  This  is  the  point  in  which  the  Darwinian 
Theory  is  so  important  to  the  History  of  the  Evolution  of 
Man  and  to  the  immediate  connection  of  the  first  part  of 
our  science,  Germ-history,  or  Ontogeny,  with  the  second 
part,  Tribal-history,  or  Phylogeny. 


CHAPTER  IV. 
THE  EARLIER  HISTORY  OF  PHYLOGENY. 

y 

JEAN  LAMARCK. 

Phylogeny  before  Darwin. — Origin  of  Species. — Karl  Linnaeus*  Idea  of 
Species,  and  Assent  to  Moses'  Biblical  History  of  Creation.— The 
Deluge.— Palaeontology.— George  Clavier's  Theory  of  Catastrophes. — 
Eepeated  Terrestrial  Revolutions,  and  New  Creations.— Lyell's  Theory 
of  Continuity.— The  Natural  Causes  of  the  Constant  Modification 
of  the  Earth. — Supernatural  Origin  of  Organisms. — Immanuel  Kane's 
Dualistic  Philosophy  of  Nature. — Jean  Lamarck. — Monistic  Philosophy 
of  Nature. — The  Story  of  his  Life.— His  Philosophie  Zoologique.— First 
Scientific  Statement  of  the  Doctrine  of  Descent.  —  Modification  of 
Organs  by  Practice  and  Habit,  in  Conjunction  with  Heredity. — Applica- 
tion  of  the  Theory  to  Man.— Descent  of  Man  from  the  Ape.— Wolfgang 
Goethe.— His  Studies  in  Natural  Science.— His  Morphology.  —  His 
Studies  of  the  "Formation  and  Transformation  of  Organisms." — 
Goethe's  Theory  of  the  Tendency  to  Specific  Differences  (Heredity 
and  of  Metamorphosis  (Adaptation). 

*'  It  wonld  be  an  easy  task  to  show  that  the  characteristics  in  the  organi. 
ration  of  man,  on  account  of  which  the  human  species  and  races  are 
grouped  as  a  distinct  family,  are  all  results  of  former  changes  of  occu- 
pation, and  of  acquired  habits,  which  have  come  to  be  distinctive  of  indi- 
viduals of  his  kind.  When,  compelled  by  circumstances,  the  most  highly 
developed  apes  accustomed  themselves  to  walking  erect,  they  gained 
the  ascendant  over  the  other  animala.  The  absolute  advantage  they 


NEW   ERA   BEGUN    BY   DARWIN.  7 1 

tmjoyed,  and  the  new  requirements  imposed  on  them,  made  them  change 
their  mode  of  life,  which  resulted  in  the  gradual  modification  of  their 
organization,  and  in  their  acquiring  many  new  qualities,  and  among  them 
the  wonderful  power  of  speech." — JEAN  LAMAECK  (1809). 

THOSE  researches  into  the  history  of  the  individual  evolution 
of  man  and  animals,  the  history  of  which  we  surveyed  in 
the  last  two  chapters,  had  until  recently  hardly  any  other 
object  than  that  of  practically  determining  the  changes  of 
form  undergone  by  the  organism  in  the  course  of  its  growth. 
But  until  within  the  past  fifteen  years,  no  one  dared  to 
seek  for  the  causes  of  these  phenomena.  During  the  entire 
century,  from  the  year  1759,  the  date  of  the  publication  of 
Wolffs  Theoria  Generations,  until  the  year  1859,  when 
Darwin  published  his  "  Origin  of  Species,"  the  causes  of 
the  evolution  of  the  germ  remained  entirely  hidden. 
During  the  whole  century  nobody  thought  of  seriously  ex- 
amining the  real  causes  of  the  changes  of  form  which  take 
place  in  the  evolution  of  the  animal  organism.  Indeed, 
the  task  was  looked  upon  as  so  difficult  that  it  entirely 
surpassed  the  powers  of  human  comprehension.  It  was 
reserved  for  Charles  Darwin  to  declare  all  these  causes. 
We  may  therefore  point  to  this  gifted  naturalist,  who, 
in  other  respects,  has  effected  a  complete  revolution 
throughout  the  whole  range  of  Biology,  as  the  founder  of 
a  new  era  in  the  field  of  Ontogeny  also.  It  is  true  that 
Darwin  himself  has  not  really  entered  very  deeply  into 
embryological  investigations,  and  even  in  his  well-known 
chief  work  on  the  phenomena  of  individual  evolution  has 
but  casually  touched  upon  these,  yet,  by  his  reform  of  the 
Theory  of  Descent,  and  by  constructing  what  he  has  named 
the  Theory  of  Selection,  he  has  placed  in  our  hands  the 


72  THE   EVOLUTION   OF  MAN. 

means  of  tracing  the  causes  of  the  Evolution  of  Forms.  It 
seems  to  me  that  it  is  in  this  respect  that  this  great  naturalist 
has  had  such  an  extraordinary  effect  on  the  entire  subject  of 
the  History  of  Evolution. 

In  glancing,  as  we  must  now  do,  at  the  last  period,  but 
just  begun,  of  ontogenetic  research,  we  enter  at  the  same 
time  into  the  second  division  of  the  History  of  Evolu- 
tion, namely,  the  History  of  the  Descent,  or  Tribe 
(Phylogeny).  In  the  first  chapter  I  drew  attention  to 
the  exceedingly  important  and  intimate  causal  connec- 
tion which  exists  between  these  two  main  branches  of  the 
History  of  Evolution, — that  of  the  individual,  and  that  of 
his  ancestors.  We  stated  this  connection  in  the  funda- 
mental Law  of  Biogeny :  the  brief  Ontogeny,  or  the 
Evolution  of  the  Individual,  is  a  swift  and  contracted 
reproduction,  a  compressed  recapitulation,  of  the  Phylogeny, 
or  the  Evolution  of  the  Species.  This  proposition  in  reality 
comprises  everything  essentially  relating  to  the  causes  of 
evolution,  and  we  shall  try  everywhere,  in  the  course  of 
these  chapters,  to  establish  it,  and  to  uphold  its  truth, 
by  adducing  actual  facts  in  proof.  The  meaning  of  this 
fundamental  Law  of  Biogeny,  in  relation  to  this  causal 
significance,  is  perhaps  yet  better  expressed  as  follows : 
"  The  evolution  of  the  species,  or  tribes  (phyla),  contains, 
in  the  functions  of  heredity  and  adaptation,  the  determin- 
ing cause  of  the  evolution  of  individual  organisms ; "  or, 
quite  briefly :  '"  Phylogeny  is  the  mechanical  cause  of 
Ontogeny." 

It  is  owing  to  Darwin  that  we  are  now  able  to  trace 
the  causes  of  individual  evolution,  which  were  previously 
deemed  quite  unapproachable,  and  to  understand  their  real 


LINN^US'    "SYSTEMA   NATURE."  73 

nature ;  we  therefore  give  his  name  to  the  new  era  of  the 
History  of  Evolution.  But  before  considering  the  grand 
discovery  by  means  of  which  Darwin  enabled  us  to  under- 
f  land  the  causes  of  evolution,  we  must  glance  rapidly  at  the 
efforts  made  by  earlier  naturalists  in  the  same  direction. 
The  historical  survey  of  these  endeavours  will  be  much 
shorter  even  than  that  of  the  labours  in  the  field  of  On- 
togeny. There  are  really  but  few  names  to  be  mentioned. 
At  the  head  stands  the  great  French  naturalist,  Jean 
Lamarck,  who,  in  1809,  for  the  first  time  gave  a  scientific 
value  to  the  so-called  Theory  of  Descent.  But  even  before 
this,  the  most  important  German  philosopher,  Kant,  and 
the  greatest  German  poet,  Goethe,  had  both  entertained 
the  idea.  During  the  previous  half-century,  however,  their 
statements  on  this  matter  remained  almost  unnoticed.  It 
was  only  in  the  commencement  of  our  century  that  "Natural 
Philosophy  "  took  up  the  question.  Previously  no  one  even 
dared  to  inquire  seriously  into  the  Origin  of  Species,  which, 
properly  speaking,  is  the  culminating  point  of  the  History 
of  Descent,  or  Phylogeny. 

The  entire  Phylogeny  of  Man,  and  also  of  other  animals, 
is  most  intimately  connected  with  the  question  as  to  the 
nature  of  species,  and  with  the  problem,  how  the  distinct 
kinds  of  animals,  which  in  systems  are  called  species,  really 
originated.  The  idea  of  species  occupies  the  foreground. 
This  idea  was  first  presented  by  Linnaeus,  who,  in  1735, 
in  iis  Sy sterna  Naturce,  attempted  the  first  accurate  dis- 
crimination and  nomenclature  of  animal  and  vegetable 
species,  and  made  a  systematic  list  of  the  species  then 
known.  Since  that  time  species  has  retained  its  place 
in  descriptive  Natural  History,  in  systematic  Zoology  and 


74  THE   EVOLUTION   OF  MAN. 

Botany,  as  the  most  important  collective  term,  although 
incessant  strife  has  been  waged  as  to  the  particular  meaning 
of  the  term.  Linnaeus  himself  gave  no  clear,  scientific  defi- 
nition of  the  real  nature  of  organic  kind,  or  species.  On  the 
contrary,  he  took  as  a  basis  the  mythological  views  of  this 
subject,  which  the  prevailing  religious  "  faith,"  grounded  on 
the  Mosaic  History  of  Creation,  had  introduced,  and  which 
are  even  now  very  generally  maintained.  He  even  adhered 
directly  to  the  Mosaic  History  of  Creation,  and  assumed 
that,  as  it  is  written  in  Genesis  "  male  and  female  created 
he  them,"  there  had  originally  been  but  one  pair  of  each 
animal  and  vegetable  kind,  or  species.  He  supposed  that 
all  the  individuals  of  a  kind  were  descendants  of  the 
original  pair  created  on  the  sixth  day  of  Creation.  Lin- 
naeus held  that  only  a  single  individual  was  created  of 
those  organisms  which  are  hermaphrodite,  that  is,  which 
unite  in  their  bodies  both  sexual  organs,  for  these  already 
possessed  in  themselves  the  qualifications  for  propagating 
their  own  species.  In  further  developing  these  mytho- 
logical ideas,  Linnaeus  adhered  to  the  Mosaic  account 
and  utilized  the  so-called  "  Deluge,"  and  the  myth  of  the 
ark  of  Noah  connected  with  it,  to  explain  the  choiology 
of  organisms,  the  doctrine,  that  is,  of  the  geographical  and 
topographical  distribution  of  animal  and  vegetable  species. 
In  harmony  with  Moses  he  assumed  that  all  plants,  animals, 
and  human  beings  had  been  destroyed  by  the  Deluge,  with 
the  exception  of  a  single  pair,  which  was  saved  in  the  ark 
to  perpetuate  the  species,  and  which  was  put  on  land  on 
Mount  Ararat  after  the  waters  had  subsided.  Mount 
Ararat  seemed  to  him  specially  adapted  for  this  disembark- 
ation, because  it  is  in  a  warm  climate  and  rises  to  a  height 


CUVIER'S   SYSTEM.  ^5 

of  more  than  sixteen  thousand  feet,  so  that  in  its  several 
zones  of  elevation  it  possessed  all  the  climates  necessary 
for  the  preservation  of  the  various  species  of  animals.  The 
animals  used  to  a  cold  climate  could  climb  to  the  highest 
parts  of  the  mountain ;  those  accustomed  to  a  warm  climate 
could  descend  to  the  foot ;  and  those  from  temperate  zones 
could  occupy  the  intermediate  portions.  From  this  moun- 
tain the  animal  and  vegetable  species  could  spread  anew 
over  the  face  of  the  earth.'25 

A  scientific  development  of  the  History  of  Creation  was 
impossible  in  the  time  of  Linnaeus,  because,  among  other 
reasons,  the  science  of  petrifactions,  or  Palaeontology,  one 
of  its  principal  bases,  did  not  as  yet  exist.  This  science 
of  petrifactions,  or  of  the  remains  of  extinct  animals  and 
plants,  is  most  intimately  connected  with  the  whole 
History  of  Creation.  Without  reference  to  it,  it  is  impos- 
sible to  answer  the  question  as  to  the  manner  in  which  the 
animals  and  plants  now  in  existence  came  into  being.  But 
the  knowledge  of  these  petrifactions  arose  in  much  later 
times,  and  the  real  founder  of  Palaeontology,  as  a  science, 
was  the  eminent  zoologist,  George  Cuvier,  who  followed 
Linnaeus  in  constructing  a  System  of  Animals,  and  who, 
in  the  beginning  of  this  century,  brought  about  a  com- 
plete reform  of  Systematic  Zoology.  The  influence  of  this 
celebrated  naturalist,  who  displayed  an  especially  great 
power  with  extraordinary  results  during  the  first  thirty 
years  of  this  century,  was  so  great  that  he  opened  new 
paths  in  almost  every  branch  of  Zoology,  but  especially  in 
Classification,  Comparative  Anatomy,  and  Palaeontology. 
It  is,  therefore,  important  to  glance  at  his  views  of  the 
nature  of  species.  In  this  respect  he  followed  Linnaeus  and 


76  THE  EVOLUTION  OF  MAN. 

the  Mosaic  account  of  Creation,  though  it  was  very  difficult 
for  him  to  do  so,  on  account  of  the  knowledge  which  he  had 
of  fossil  animal  forms.  He  was  the  first  to  show  clearly 
that  a  number  of  totally  different  series  of  inhabitants  had 
lived  on  our  globe.  He  also  showed  that  we  must  dis- 
tinguish at  least  ten  or  fifteen  different  main  periods  in  the 
history  of  the  earth,  each  of  which  exhibits  a  series  of 
animals  and  plants  of  its  own,  peculiar  to  itself. 

Of  course,  Cuvier  was  at  once  confronted  with  the  ques- 
tion, whence  these  various  series  of  inhabitants  had  come, 
and  whether  they  had  any  connection  with  each  other. 
He  answered  this  question  negatively,  and  maintained  that 
these  several  "  creations  "  were  totally  independent  of  each 
other;  hence,  that  the  supernatural  act  of  creation  by  which, 
according  to  the  received  account  of  creation,  the  animal 
and  vegetable  species  came  into  being,  was  repeated  several 
times.  Consequently,  a  series  of  quite  distinct  periods  of 
creation  must  have  followed  one  another,  and  in  connection 
with  them  there  must  have  occurred  several  vast  alterations 
of  thewhole  surface  of  the  earth, — revolutions  and  cataclysms 
similar  to  the  mythical  Flood.  These  catastrophes  and 
upheavals  were  favourite  subjects  with  Cuvier ;  especially 
as  at  that  time  the  science  of  geology  was  also  beginning 
to  move  greatly,  and  made  rapid  progress  towards  a  know- 
ledge of  the  structure  and  origin  of  the  earth.  Others, 
especially  the  geologist  Werner  and  his  school,  were  occupied 
in  carefully  examining  the  various  layers  of  the  crust  of  the 
earth,  and  systematically  investigating  the  fossils  found 
in  these.  The  result  of  their  researches  also  was  the  recog- 
nition of  several  periods  of  creation.  The  inorganic  crust 
of  the  earth,  the  stratified  surface,  bore  evidence  of  having 


THEORY   OF   CATASTROPHES.  77 

been  just  as  different  at  every  period  as  were  the  animals 
and  plants  then  inhabiting  it.  Combining  this  view  with  the 
results  of  his  own  palaeontological  and  zoological  researches, 
and  striving  to  understand  clearly  the  whole  course  of  the 
evolution  of  Creation,  Cuvier  arrived  at  the  hypothesis 
usually  called  the  Theory  of  Cataclysms  or  Catastrophes,  or 
the  Doctrine  of  Violent  Upheavals.  According  to  it  several 
revolutions  occurred  on  our  earth  at  certain  times,  suddenly 
destroying  every  living  inhabitant ;  and  at  the  end  of  each 
of  these  catastrophes  an  entirely  new  creation  of  organisms 
took  place.  But  as  the  latter  cannot  be  conceived  as 
having  been  effected  wholly  by  natural  means,  we  must 
suppose,  in  explanation,  that  the  Creator  supernaturally 
interfered  in  the  natural  course  of  things.  This  Doctrine  of 
Revolutions,  treated  by  Cuvier  in  a  separate  work,  which 
has  been  translated  into  several  modern  languages,  was 
soon  generally  accepted,  and  for  half  a  century  continued 
to  prevail  among  biologists ;  there  are  even  yet  a  few 
prominent  naturalists  who  advocate  it. 

It  is  true  that  more  than  forty  years  ago  Cuvier's 
Doctrine  of  Catastrophes  was  altogether  renounced  by 
geologists ;  and  first  of  all  by  the  English  geologist,  Charles 
Lyell,  the  most  important  authority  in  this  branch  of 
natural  science.  As  early  as  the  year  1830,  in  his  famous 
"Principles  of  Geology,"  he  proved  that  that  doctrine  is 
utterly  false  so  far  as  the  crust  of  the  earth  itself  is  con- 
cerned ;  and  he  showed  that  in  order  to  explain  the  structure 
and  evolution  of  mountains,  there  is  no  need  of  having  re- 
course to  supernatural  causes  or  universal  catastrophes.  On 
the  contrary,  the  ordinary  causes  which  even  now  unceasingly 
effect  the  transformation  and  reconstruction  of  the  earth,  are 


78  THE   EVOLUTION   OF  MAN. 

amply  sufficient  to  explain  these  phenomena.  These  causes 
are :  atmospheric  influences ;  water  in  its  various  forms — 
such  as  snow  and  ice,  fog  and  rain,  the  running  stream 
and  the  surging  sea;  and  finally,  the  volcanic  phenomena 
contributed  by  the  hot  liquid  mass  in  the  interior  of  the 
earth.  The  most  convincing  proof  was  furnished  by  Lyell, 
that  these  natural  causes  are  quite  sufficient  to  explain  all  the 
phenomena  of  the  structure  and  development  of  the  crust 
of  the  earth.  The  geological  teaching  of  Cuvier  as  to  the 
revolutions  and  new  creations  was,  therefore,  soon  totally 
abandoned,  but  in  Biology  the  doctrine  prevailed  unopposed 
for  thirty  years  longer.  Zoologists  and  botanists,  as  far  as 
they  at  all  permitted  themselves  to  think  on  the  origin  of 
organisms,  adhered  to  Cuvier's  false  doctrine  of  repeated 
new  creations  and  re-formations  of  the  earth.  This  is  cer- 
tainly one  of  the  most  curious  examples  of  two  closely 
related  sciences  long  pursuing  utterly  divergent  courses. 
One — Biology — remains  far  behind  in  the  dualistic  path, 
and  even  denies  the  possibility  of  solving  "questions  of 
creation  "  by  the  study  of  natural  phenomena.  The  other — 
Geology — moves  far  ahead  in  the  monistic  path,  and  solves 
those  very  questions  by  the  discovery  of  the  actual  causes. 

As  an  instance  how  utterly  biologists  refrained  from  in- 
quiries into  the  origin  of  organisms,  and  the  creation  of  the 
animal  and  vegetable  species,  during  this  period  from  1830 
to  1859,  I  mention,  from  my  own  experience,  the  fact  that 
during  all  the  whole  course  of  my  studies  at  the  university, 
I  never  heard  a  single  word  on  these  most  important  and 
fundamental  questions  of  biology.  During  this  time,  from 
1852  to  1857, 1  had  the  good  fortune  to  listen  to  the  most 
distinguished  teachers  in.  all  branches  of  the  science'  of 


CONSERVATISM   OF   BIOLOGY.  79 

organic  nature;  but  not  one  of  them  ever  spoke  of  this 
fundamental  point,  or  even  once  alluded  to  the  question  of 
the  origin  of  species.  Not  a  word  was  ever  spoken  in 
reference  to  the  earlier  attempts  toward  understanding  the 
origin  of  the  animal  and  vegetable  species;  it  was  never 
thought  worth  while  to  allude  to  Lamarck's  valuable 
Philosophie  Zoologique,  in  which  that  attempt  had  been 
made  in  the  year  1809.  The  enormous  opposition  which 
Darwin  met  with  when  he  first  took  up  this  question 
again  may,  therefore,  be  understood.  His  attempt  seemed 
at  first  to  be  unsubstantial  and  unsupported  by  previous 
labours.  Even  in  1859  the  entire  problem  of  creation,  the 
whole  question  of  the  origin  of  organisms,  was  considered 
by  biologists  as  supernatural  and  transcendental.  Even  in 
speculative  philosophy,  in  which  this  question  should 
necessarily  be  approached  from  various  sides,  no  one  dared 
to  take  it  seriously  in  hand. 

The  dualistic  position  taken  by  Immanuel  Kant,  and  the 
extraordinary  importance  attached,  during  the  whole  of  this 
century,  to  this  most  influential  of  modern  philosophers, 
probably  offer  the  best  explanation  of  the  last-mentioned 
fact.  For  while  this  great  genius,  equally  excellent  as  n 
naturalist  and  a  philosopher,  in  the  field  of  inorganic  nature 
aided  essentially  in  constructing  a  "Natural  History  of 
Creation,"  he  for  the  most  part  adopted  the  supernatural 
view  of  the  origin  of  organisms.  On  the  one  hand,  Kant, 
in  his  "  Universal  History  of  Nature  and  Theory  of 
the  Heavens,"  made  a  most  successful  and  important  "at- 
tempt to  treat  the  constitution  and  the  mechanical  origin 
of  the  entire  universe  according  to  Newtonian  principles," 
or,  in  other  words,  to  treat  it  mechanically,  to  conceive 


80  THE   EVOLUTION   OF   MAN. 

it  monistically :  and  this  attempt  of  his  to  explain  the 
origin  of  the  entire  world  by  means  of  naturally  working 
causes  (causce  ejfidentes),  forms  to  this  day  the  basis  of 
all  our  natural  cosmogony.  But,  on  the  other  hand,  Kant 
maintained  that  the  "  principle  of  the  mechanism  of  nature 
here  applied,  without  which,  after  all,  there  could  be  no 
science  of  nature,"  was  wholly  inadequate  to  explain  the 
phenomena  of  organic  nature,  and  especially  the  origin  of 
organisms;  that  it  was  necessary  to  assume  supernatural 
causes  effecting  a  design  (causce  finales)  for  the  origin  of 
these  natural  bodies  constructed  with  design.  Indeed,  he 
even  went  so  far  as  to  assert  that  "it  is  quite  certain 
we  cannot  become  adequately  acquainted  with  organized 
beings,  and  their  inner  possibilities,  by  purely  mechanical 
principles  of  nature,  much  less  are  we  able  to  explain 
them ;  and  that  this  is  so  much  the  case  that  we  may  boldly 
assert  that  it  is  not  rational  for  man  even  to  enter  upon 
such  speculations,  or  to  expect  that  a  Newton  will  ever 
arise  who,  by  natural  laws  not  ordered  by  design,  can 
render  the  production  of  a  blade  of  grass  intelligible ;  in 
fact,  we  are  compelled  utterly  to  deny  that  it  is  possible 
for  man  ever  to  reach  such  knowledge."  In  these  words 
Kant  most  definitely  declared  the  dualistic  and  teleological 
standpoint  which  he  adopted  in  the  science  of  organic 
nature. 

Kant  sometimes,  however,  departed  from  this  stand- 
point, especially  in  some  very  remarkable  passages  which 
I  have  discussed  at  some  length  in  the  fifth  chapter  of  my 
"  History  of  Creation,"  in  which  he  has  expressed  himself 
in  quite  the  opposite,  or  monistic  sense.  With  reference  tc 
these  passages,  as  I  there  showed,  he  might  even  be  declared 


KANT.  8l 

an  adherent  of  the  Theory  of  Descent.  Several  very  sig- 
nificant expressions,  to  which  Fritz  Schultze,  in  his  interest 
ing  work  on  "  Kant  and  Darwin,26  has  lately  again  called 
attention,  actually  enable  us  to  recognize  Kant27  as  the 
earliest  prophet  of  Darwinism.  He  expresses  with  perfect 
c3  aarness  the  great  idea  of  an  all-embracing,  uniform  evolu- 
tion ;  he  assumes  "  a  variation  from  the  primitive  type  of 
the  tribe  as  the  result  of  natural  wandering."  He  even 
declares  that  man  originally  moved  on  four  feet,  and  that 
it  was  only  gradually  that  the  human  race  raised  their 
heads  proudly  over  those  of  their  old  comrades,  the  beasts. 
But  all  these  evidently  monistic  utterances  are  but  stray 
rays  of  light ;  as  a  rule  Kant  adhered  in  Biology  to 
those  obscure  dualistic  notions  according  to  which  the 
powers  which  operate  in  organic  nature  are  entirely 
different  from  those  which  prevail  in  the  inorganic  world. 
This  dualistic,  or  two-sided  conception  of  nature  is  still 
dominant  in  school-philosophy ;  most  philosophers  still 
consider  these  two  domains  of  natural  phenomena  as 
entirely  different.  On  one  side  is  the  field  of  inorganic 
nature,  the  so-called  "inanimate"  world,  where  only 
mechanical  laws  (causes  efficientes)  are  supposed  to  operate, 
of  necessity  and  without  purpose.  On  the  other  side  is 
the  field  of  "  animated "  organic  nature,  all  the  phenomena 
of  which  in  their  profoundest  essence  and  first  origin  can 
be  made  intelligible  only  by  assuming  pre-ordained  pur- 
poses, or  so-called  (causce  finales)  causes  fulfilling  a  design. 

Although  the  question  of  the  origin  of  animal  and 
•vegetable  species,  and  the  allied  question  as  to  the  creation 
of  man,  remained  until  the  year  1859  under  the  sway  of 
these  false  dualistic  prejudices,  and  were  very  generally 


82  TIIE   EVOLUTION   OF   MAN. 

declared  to  be  a  subject  beyond  the  reach  of  scientific 
knowledge,  yet  even  in  the  beginning  of  our  century  there 
were  independent  eminent  minds,  who,  undeterred  by  the 
prevailing  doctrines,  took  these  questions  quite  seriously  in 
hand.  The  so-called  earlier  school  of  Natural  Philosophy, 
which  has  so  often  been  abused,  deserves  the  highest  praise 
in  this  respect.  It  was  represented  in  France  by  Jean 
Lamarck,  Buffon,  Geoffroy  St.  Hilaire,  and  Ducrotay  Blain- 
ville ;  in  Germany,  by  Wolfgang  Goethe,  Eeinhold  Trevi- 
ranus,  Schelling,  and  Lorenz  Oken. 

The  gifted  naturalist  and  philosopher  who  must  here 
be  mentioned  first,  is  Jean  Lamarck.  He  was  born  at 
Bazentin,  in  Picardy,  August  1,  1744,  and  was  the  son  of 
a  clergyman  who  destined  him  for  the  Church.  He,  how- 
ever, first  joined  the  army,  and  as  a  boy  of  sixteen  dis- 
tinguished himself  by  his  bravery  in  the  battle  of  Lippstadt 
in  Westphalia,  which  resulted  unfavourably  for  the  French. 
He  was  then  stationed  for  several  years  in  a. garrison  in 
the  south  of  France.  Here  he  became  acquainted  with 
the  interesting  flora  on  the  Mediterranean  coast,  which 
soon  won  him  over  to  the  study  of  botany.  He  resigned 
his  commission,  and  published,  as  early  as  the  year  1778, 
his  valuable  Flore  Frangaise.  For  years  he  could  gain  no 
scientific  position.  It  was  only  in  his  fiftieth  year,  in  1794, 
that  he  obtained  a  poor  professorship  of  zoology  at  the 
museum  of  the  Jardin  de  Plantes  in  Paris.  His  position 
caused  him  to  enter  more  deeply  into  the  study  of  zoology, 
towards  the  classification  of  which  his  labours  were  as 
valuable  and  important  as  those  which  he  had  dedicated 
to  systematic  botany.  In  1302  he  published  his  Considera- 
tions swr  les  corps  vivants,  which  contains  the  first  germs  oj 


HISTORY   OF   LAMARCK.  83 

his  Theory  of  Descent.  In  1809  appeared  the  important 
Philosophie  Zoologique,  the  principal  work  in  which  he 
elaborated  this  theory.  In  1815  he  gave  to  the  world  his 
comprehensive  treatise  on  the  Natural  History  of  Inver- 
tebrates (Histoire  naturelle  des  animaux  sans  vertebres\ 
in  the  Introduction  to  which  the  same  theory  is  again 
developed.  About  this  time  Lamarck  entirely  lost  his  eye- 
sight. Grudging  fate  never  favoured  him.  While  his 
principal  opponent,  Cuvier,  was  lucky  enough  to  gain  an 
influential  position  and  the  highest  rank  of  scientific  fame 
in  Paris,  Lamarck,  who  far  surpassed  Cuvier  m  clear  and 
high-minded  conception  of  nature,  was  obliged  to  struggle 
in  lonely  seclusion  for  the  very  necessaries  of  life,  and  could 
obtain  no  recognition.  In  1829  his  laborious  life  closed  in 
the  midst  of  the  most  needy  circumstances.28 

Lamarck's  Philosophie  Zoologique  was  the  first  scientific 
outline  of  a  real  history  of  the  evolution  of  Species,  a 
natural  history  of  the  creation  of  plants,  animals,  and 
men.  The  effect  produced  by  this  remarkable  and  im- 
portant book  was,  like  that  of  Wolff's,  none  :  neither  was 
understood.  No  naturalist  felt  called  upon  to  interest  him- 
self seriously  in  this  book,  and  to  forward  the  development 
of  the  rudiments  of  the  most  valuable  progress  in  Biology 
which  it  laid  down,  The  most  eminent  botanists  and 
zoologists  threw  the  book  entirely  aside,  and  did  not  con- 
sider it  worth  refuting.  Cuvier,  who  taught  and  laboured 
in  Paris  as  a  contemporary  of  Lamarck,  in  his  account  of 
the  progress  made  in  Natural  Science,  in  which  the  most 
insignificant  observations  were  mentioned,  did  not  deem  it 
worth  while  to  devote  a  syllable  to  this  the  greatest  advance. 
In  short,  Lamarck's  Zoological  Philosophy  shared  the  fate 


84  THE   EVOLUTION   OF   MAN. 

of  Wolffs  Theory  of  Evolution,  and  was  ignored  for  half  a 
century.  Even  Oken  and  Goethe,  the  German  natural 
philosophers,  who  were  simultaneously  employed  in  similar 
speculations,  do  not  appear  to  have  been  aware  of  Lamarck's 
work.  Had  they  known  it,  it  would  have  been  a  great 
help  to  them,  and  they  would  have  worked  out  the  Theory 
of  Evolution  to  a  point  beyond  that  which  was  otherwise 
possible  to  them. 

To  enable  my  readers  to  judge  of  the  great  value  of  the 
Philosophic  Zoologique,  I  shall  here  briefly  mention  some  of 
the  most  important  of  Lamarck's  ideas.  According  to  him. 
there  is  no  essential  difference  between  animate  and  inani- 
mate nature;  all  nature  is  a  single  world  of  connected 
phenomena,  and  the  same  causes  which  form  and  trans- 
form inanimate  natural  bodies  are  alone  those  which  are  at 
work  in  animate  nature.  Hence,  we  must  apply  the  same 
methods  of  investigation  and  explanation  to  both.  Life  is 
only  a  physical  phenomenon.  The  conditions  of  internal 
and  external  form  of  all  orgaoisms — plants  and  animals, 
with  man  at  their  head — are  to  be  explained,  like  those  of 
minerals  and  other  inanimate  natural  bodies,  only  by 
natural  causes  (causce  efficientes),  without  the  addition  of 
purposive  causes  (causce  finales).  The  same  is  true  of  the 
origin  of  the  various  species.  Without  contradicting  nature, 
v^e  can  neither  assume  for  them  one  original  act  of  crea- 
tion, nor  repeated  new  creations  as  implied  in  Cuvier's 
Doctrine  of  Catastrophes, — but  only  a  natural,  uninterrupted, 
and  necessary  evolution.  The  entire  course  of  the  evolu- 
tion of  the  earth  and  its  inhabitants  is  continuous  and 
connected.  All  the  various  species  of  animals  and  plants 
vvluch  we  now  see  around  us,  or  which  ever  existed,  have 


LAMARCK'S  THEORIES.  85 

developed  in  a  natural  manner  from  previously  existing, 
different  species ;  all  are  descendants  of  a  single  ancestral 
form,  or  at  least  of  a  few  common  forms.  The  most  ancient 
ancestral  forms  must  have  been  very  simple  organisms  of 
the  lowest  grade,  and  must  have  originated  from  inorganic 
matter  by  means  of  spontaneous  generation.  Adaptation 
through  practice  and  habit,  to  the  changing  external  condi- 
tions of  life,  has  ever  been  the  cause  of  changes  in  the  nature 
of  organic  species,  and  Heredity  caused  the  transmission  of 
these  modifications  to  their  descendants. 

These  are  the  principal  outlines  of  the  theory  of 
Lamarck,  now  called  the  Theory  of  Descent  or  Transmuta- 
tion, and  to  which,  fifty  years  later,  attention  was  again 
called  by  Darwin,  who  firmly  supported  it  with  new  proofs. 
Lamarck,  therefore,  is  the  real  founder  of  this  Theory  of 
Descent  or  Transmutation,  and  it  is  a  mistake  to  attribute 
its  origin  to  Darwin.  Lamarck  was  the  first  to  formulate 
the  scientific  theory  of  the  natural  origin  of  all  organisms, 
including  man,  and  at  the  same  time  to  draw  the  two  ulti- 
mate inferences  from  this  theory:  firstly,  the  doctrine  of 
the  origin  of  the  most  ancient  organisms  through  spon- 
taneous generation;  and  secondly,  the  descent  of  Man 
from  the  Mammal  most  closely  resembling  Man — the  Ape. 

Lamarck  attempted  to  explain  the  latter  process,  a  most 
important  one,  and  of  special  interest  to  us  here,  by  the 
same  efficient  causes  to  which  he  had  also  referred  the 
natural  origin  of  animal  and  vegetable  species.  He  con 
sidered  that,  on  the  one  hand,  practice  and  habit  (Adapta- 
tion), and,  on  the  other,  Heredity,  are  the  most  important 
of  these  causes.  The  chief  modifications  of  the  organs 
of  animals  and  plants  result,  according  to  him,  from  the 


86  THE   EVOLUTION    OF   MAN. 

functions  or  actions  of  the  organs  themselves,  from  the 
exercise  or  absence  of  exercise,  the  use  or  disuse  of  these 
organs.  To  mention  examples,  the  Woodpecker  and  the 
Humming-bird  owe  their  peculiarly  long  tongue  to  their 
habit  of  using  these  organs  to  take  their  food  out  oi 
narrow  and  deep  crevices;  the  Frog  acquired  a  web  between 
its  toes  from  the  motions  of  swimming;  the  Giraffe  gained 
its  long  neck  by  stretching  it  up  to  the  branches  of  trees. 
Habits,  the  use  and  disuse  of  organs,  are  certainly  of  the 
greatest  importance  as  efficient  causes  of  organic  form ;  but 
they  are  insufficient  to  explain  the  modification  of  species. 
As  a  second  and  equally  important  cause,  Lamarck  fully 
perceived  that  Heredity  must  necessarily  co-operate  with 
Adaptation.  He  maintained  that  the  variations  of  organs 
arising  from  habit  or  use  are  in  themselves  at  first  but 
insignificant  in  each  separate  individual ;  but  that  by  the 
accumulation  of  the  effects  produced  in  each  individual, 
transmitted  from  generation  to  generation  in  an  ever  increas- 
ing number,  they  become  very  significant.  This  was  quite 
a  correct  fundamental  idea ;  but  Lamarck  did  not  reach  the 
principle  which  Darwin  subsequently  introduced  as  the 
most  important  factor  in  the  Theory  of  Transmutation, 
namely,  the  principle  of  Natural  Selection  in  the  Struggle 
for  Existence.  Lamarck  failed  to  discover  this  most  im- 
portant causal  relation,  and  this,  together  with  the  low 
condition  of  all  biological  sciences  at  that  time,  prevented 
him  from  more  firmly  establishing  his  theory  of  the  common 
descent  of  animals  and  man. 

Lamarck  also  attempted  to  explain  the  evolution  of  Man 
from  the  Ape,  as  principally  due  to  the  progress  made  by 
the  Ape  in  its  habits  of  life,  the  further  development  and 


LAMARCK   ON   THE  "APE   QUESTION."  8? 

increased  use  of  its  organs,  and  to  the  fact  that  it  trans- 
mitted the  improvements  thus  acquired  to  its  descend- 
ants. Lamarck  considered  the  most  important  of  these  ad- 
vantageous variations  to  be  the  erect  gait  of  Man,  the  differ- 
ing form  of  the  hands  and  feet,  the  growth  of  language, 
and  the  correlative  higher  development  of  the  brain.  He 
assumed  that  the  Apes  most  closely  akin  to  Man,  those 
which  became  the  ancestors  of  mankind,  made  the  first 
step  toward  becoming  human  when  they  gave  up  the  habit 
of  climbing  and  living  on  trees,  and  accustomed  themselves 
to  an  upright  gait.  This  resulted  in  the  carriage  peculiar 
to  Man  and  in  the  reconstruction  of  the  spinal  column  and 
pelvis,  as  well  as  in  the  specialization  of  the  two  pairs  of  limbs 
— the  fore  pair  developing  into  hands  for  the  purpose  of 
grasping  and  touching,  while  the  hind  pair  were  used  only 
for  walking,  and  thus  developed  into  true  feet.  In  con- 
sequence of  the  totally  changed  mode  of  life  and  of  the 
correlation  and  interrelation  of  the  various  parts  of  the 
body  and  their  functions,  important  changes  occurred  also 
in  other  organs  and  their  functions.  The  change  of  food, 
for  example,  caused  a  change  in  the  jaws  and  teeth,  and, 
consequently,  in  the  entire  formation  of  the  face.  The  tail, 
no  longer  used,  gradually  disappeared.  As  these  Apes  lived 
together  in  societies  and  acquired  regulated  family  relations, 
such  as  are  still  found  among  the  higher  classes  of  Apes,  the 
social  habits,  or  so-called  "  social  instincts,"  were  especially 
developed.  The  Ape's  language  of  mere  soimds  grew  into 
the  word-language  of  Man,  and  abstract  ideas  were  accu- 
mulated from  concrete  impressions.  The  brain  gradually 
developed  in  correlation  with  the  larynx ;  the  organ  of  the 
mind  in  interrelation  with  that  of  speech.  These  important 


88  THE   EVOLUTION   OF   MAN. 

ideas  of  Lamarck  contain  the  first  and  oldest  gerras  of  a 
real  history  of  the  human  tribe. 

Toward  the  end  of  the  preceding  and  the  beginning 
of  this  century,  the  great  poet  Goethe,  whose  genius 
was  of  the  highest  order,  busied  himself,  independently  of 
Lamarck,  with  the  problem  of  creation,  and  his  thoughts 
on  this  subject  are  of  special  interest.  It  is  well  known 
that  Goethe's  ready  recognition  of  all  the  beauties  of 
Nature,  and  his  deep  insight  into  her  workings,  early 
attracted  him  to  natural  scientific  studies  of  the  most 
various  kinds.  Throughout  his  life  these  formed  the 
favourite  occupation  of  his  leisure  hours.  The  theory  of 
colours  especially  resulted  in  his  well-known  and  compre- 
hensive work  on  this  subject ;  but  the  most  valuable  and 
important  of  Goethe's  natural  scientific  studies  are  those  in 
connection  with  organic  bodies,  with  "  Life,  that  splendid, 
priceless  thing."  In  Morphology,  the  doctrine  of  forms, 
he  made  most  unusually  deep  researches.  Aided  by  Com- 
parative Anatomy,  he  obtained  most  brilliant  results  in 
this,  and  went  far  in  advance  of  his  time.  His  cranial 
theory,  his  discovery  of  the  temporal  jawbone  in  man,  and 
his  doctrine  of  the  metamorphosis  of  plants,  must  be  espe- 
cially mentioned  here.29  These  morphological  studies  led 
Goethe  to  make  those  researches  into  the  formation  and 
transformation  of  organisms  which  we  must  rank,  after  those 
of  Lamarck,  among  the  oldest  and  profoundest  rudiments 
of  phylogenetical  science.  He  came  so  near  the  Theory  of 
Descent  that  he  must  be  classed  with  Lamarck  among  the 
founders  of  it.  It  is  true  that  Goethe  has  nowhere  given 
a  connected  scientific  exposition  of  his  theory  of  evolution ; 
but  his  brilliant  miscellaneous  writings, "Zw  Morphologie* 


GOETHE  AS  A  NATURALIST.  89 

abound  in  most  excellent  ideas.  Some  of  them  may  indeed 
be  called  the  rudiments  of  the  Theory  of  Descent.  In 
proof  of  this  it  is  sufficient  to  adduce  some  of  his  most 
remarkable  propositions.  He  says  :  "  This,  then,  is  what  we 
have  gained,  fearlessly  to  assert  that  the  more  perfect  natural 
organisms,  such  as  Fishes,  Amphibia,  Birds,  Mammals,  and 
Man  at  the  head  of  the  last,  have  been  formed  after  one 
primordial  type,  the  very  permanent  parts  of  which  only 
vary  a  little  one  way  or  another,  and  which  in  the  course 
of  reproduction  is  still  being  remoulded  and  perfected" 
(1796).  This  "  primordial  type  "  of  Vertebrates,  after  which 
Man  also  has  been  shaped,  answers  to  what  we  call  "  the 
common  ancestral  form  of  the  vertebrate  tribe,"  and  from 
which  all  the  various  species  of  Vertebrates  have  arisen  by 
constant  "development,  variation,  and  reproduction."  In 
another  passage  Goethe  says  (1807)  :  "  Plants  and  animals, 
regarded  in  their  most  imperfect  condition,  are  hardly  dis- 
tinguishable. This  much,  however,  we  may  say,  that  from 
a  condition  in  which  plant  is  hardly  to  be  distinguished 
from  animal,  creatures  have  appeared,  gradually  perfecting 
themselves  in  two  opposite  directions, — the  plant  is  finally 
glorified  into  the  tree,  enduring  and  motionless,  the  animal 
into  the  human  being,  of  the  highest  mobility  and  free- 
dom." 

That  Goethe,  in  these  and  other  utterances,  did  not 
apeak  merely  figuratively,  that  he  grasped  the  internal 
relation  and  connection  of  organic  forms  in  a  genealogical 
senoe,  is  yet  more  evident  in  remarkable  separate  passages  in 
which  he  declares  himself  as  to  the  causes  of  the  external 
multiplicity  of  species,  on  the  one  hand,  and  of  the  internal 
unity  of  their  structure  on  the  other.  He  assumed  that 


90  THE  EVOLUTION  OF  MAN. 

every  organism  is  the  product  of  the  co-operation  of  two 
contrary  constructive  forces,  or  formative  tendencies.  One, 
the  internal  formative  tendency,  "the  centripetal  force,"  is 
that  of  the  type,  or  "the  tendency  toward  specification," 
which  constantly  aims  at  maintaining  uniform  the  organic 
forms  of  the  species  in  the  series  of  generations.  This  is 
Heredity.  The  other,  the  external  formative  tendency, 
"  the  centrifugal  force,"  is  variation,  or  "  the  tendency 
toward  metamorphosis,"  which  acts,  through  the  continual 
changes  made  in  the  external  conditions  of  their  existence, 
so  as  continually  to  vary  the  species.  This  is  Adapta 
tion. 

In  this  significant  conception,  Goethe  very  nearly  con- 
ceived the  two  great  mechanical  factors,  Heredity  and  Adap- 
tation, whioh  are,  we  assert,  the  most  important  efficient 
causes  of  the  formation  of  species.  For  example,  he  says, 
that  "  at  the  foundation  of  all  organization  there  is  an 
original  intrinsic  kinship  "  (which  is  Heredity) ;  "  the  variety 
of  forms,  however,  is  due  to  the  conditions  of  relation 
necessarily  held  to  the  external  world,  on  account  of  which 
we  may  properly  assume,  for  the  purpose  of  explaining  the 
present  forms,  which  are  both  varied  and  unvaried,  that 
there  was  diversity,  originally  and  simultaneously,  and  that 
a  progressive  transformation  is  continually  going  on" 
(which  is  Adaptation). 

In  order  rightly  to  appreciate  Goethe's  morphological 
views  it  is,  however,  necessary  to  grasp  the  connection 
between  the  whole  peculiar  course  of  his  monistic  study  of 
nature  and  his  pantheistic  conception  of  the  world.  Most 
significant  in  this  respect  is  the  lively  and  warm  interest 
with  which  he  followed  the  efforts  which  the  French 


GOETHE   VIRTUALLY   AN   EVOLUTIONIST.  91 

natural  philosophers  were  making  in  the  same  direction, 
and  especially  the  contest  between  Cuvier  and  Geofiroy  St. 
HUaire.  (See  Chapter  IV.  in  "  History  of  Creation.")  It 
is  also  necessary  to  be  in  some  degree  master  of  Goethe's 
language  and  his  process  of  thought,  before  it  is  possible 
rightly  to  understand  the  many  expressions,  often  incidental, 
which  refer  to  the  doctrine  of  descent.  He  who  does  not 
know  the  great  poet  and  thinker  as  a  whole,  may  possibly 
even  construe  these  very  expressions  in  a  contrary  sense. 

In  proof  of  this  I  adduce  the  strange  fact  that  two 
second-rate  German  zoologists  have  recently  discovered 
that  Goethe  was  an  extremely  narrow-minded  naturalist 
and  a  "willing  adherent  of  the  doctrine  of  constancy  of 
species."  Karl  Semper,  the  gifted  discoverer  of  "Haeckelism 
in  Zoology,"  and  Hobby  Kossman,  the  ingenious  "  Solver  of 
the  Rhizo-cephalic  Problem,"  have  extracted  from  Goethe's 
morphological  writings  that  the  latter  needy  Frankfort 
geniuses  had  neither  a  clear  conception  of  the  whole  sig- 
nificance of  organic  forms,  nor  the  faintest  idea  of  the 
natural  evolution  of  these  forms,  and  of  their  connection 
by  common  descent.  All  who  know  the  poor  and  narrow- 
minded  literary  productions  of  Semper  and  Kossman  must 
smile  at  the  sentence  of  annihilation  thus  pronounced  on 
Goethe's  conception  of  nature. 

Notwithstanding  the  condemnation  by  these  great 
students  of  animal  life,  the  rest  of  the  world  may  continue 
to  admire  Goethe  as  a  true  prophet  of  the  theory  of  descent. 
The  numerous  sentences  which  I  have  prefixed,  as  mottos 
to  the  chapters  of  the  Generelle  Morphologie,  clearly 
show  how  far  Goethe  had  advanced  in  his  conception  of 
the  innate  genealogical  connection  of  the  diverse  organic 


Q2  THE   EVOLUTION   OF  MAN. 

forms.     At  the  end  of  the  last  century  he  so  nearly  grasped 
the  principles  of  natural  tribal  history,  that  we  are  justified 
in  regarding  him  as   one   of  the   earliest  forerunners   of 
Darwin,  although,  unlike   Lamarck,  he  did  not  formuiaf 
t,he  Theory  of  Descent  in  a  scientific  system. 


CHAPTER  V. 
MODERN   PHYLOGENY. 

CHARLES  DARWIN. 

Relation  of  Modern  to  Earlier  Phylogeny. — Charles  Darwin's  Work  on  the 
Origin  of  Species. — Causes  of  its  Remarkable  Success. — The  Theory  of 
Selection  :  the  Interrelation  of  Hereditary  Transmission  and  Adaptation 
in  the  Struggle  for  Existence. — Darwin's  Life  and  Voyage  Bound  the 
World. — His  Grandfather,  Erasmus  Darwin. —  Charles  Darwin's  Study 
of  Domestic  Animals  and  Plants. — Comparison  of  Artificial  with 
Natural  Conditions  of  Breeding. — The  Struggle  for  Existence. — Neces- 
sary Application  of  the  Theory  of  Descent  to  Man. — Descent  of  Man 
from  the  Ape.  —  Thomas  Huxley.— Karl  Vogt.  —  Friedrich  Rolle.— 
The  Pedigrees  in  the  Generelle  Morphologic  and  the  "  History  of 
Creation." — The  Genealogical  Alternative. — The  Descent  of  Man  from 
Apes  deduced  from  the  Theory  of  Descent. — The  Theory  of  Descent 
as  the  Greatest  Inductive  Law  of  Biology. — Foundation  of  this  Induc- 
tion.— Paleontology. — Comparative  Anatomy. — The  Theory  of  Endi. 
mentary  Organs. — Purposelessness,  or  Dysteleology. — Genealogy  of  the 
Natural  System. — Chorology. — CEkology. — Ontogeny. — Refutation  of 
the  Dogma  of  Species. — The  "  Monograph  on  the  Chalk  Sponges  ; " 
Analytic  Evidence  for  the  Theory  of  Descent. 

"  By  considering  the  embryological  structure  of  man — the  homologies 
which  he  presents  with  the  lower  animals — the  rudiments  which  he  retains — 
aud  the  reversions  to  which  he  is  liable,  we  can  par.ly  recall  in  imagination 
the  former  condition  of  our  early  progenitors ;  and  can  approximately  place 
them  in  their  proper  position  in  the  zoological  series.  We  thus  learn  that 
man  is  descended  from  a  hairy  quadruped,  furnished  with  a  tail  and  pointed 
ears,  probably  arboreal  in  its  habits,  and  an  inhabitant  of  the  Old  World. 


94  THE   EVOLUTION    OF  MAN. 

This  creature,  if  its  whole  structure  had  been  examined  by  a  naturalist, 
would  have  been  classed  among  the  Quadrumana,  as  surely  as  would  the 
common  and  still  more  ancient  progenitor  of  the  Old  and  New  World 
monkeys."— CHARLES  DABWIN  (1871). 

[N  the  short  time  that  has  passed  since  the  appearance  of 
Charles  Darwin's  book  "  On  the  Origin  of  Species  in  the 
Animal  and  Vegetable  Kingdom,"  the  History  of  Evolution 
has  advanced  so  greatly  that  it  is  scarcely  possible  to  point 
to  an  equally  great  advance  throughout  the  whole  record 
of  the  Natural  Sciences.     The  literature  of  Darwinism  is 
increasing  day  by  day,  not  only  in  connection  with  Zoology 
and  Botany — which  are  the  special  sciences  most  affected 
and  reformed  by  the  Darwinian  Theory — but  far  beyond. 
It  is  applied  in  much  wider  circles  with  a  zeal  and  interest 
which  no  other  scientific  theory  has  ever  aroused.     There 
are  two  distinct  circumstances  which   principally  explain 
this   extraordinary   success.     In    the    first    place,   all    the 
natural  sciences,  and  especially  Biology,  made  unusually 
rapid  progress  during  the  preceding  half  century,  and  from 
actual  experience  many  new  data  for  the  theory  of  natural 
evolution  were  amassed.     When  compared  with  the  failure 
of  Lamarck,  and  the  earlier  naturalists  to  obtain  recognition 
for  their  first   attempts  to   explain  the  origin  of  organ- 
isms and  of  man,  the  success  of  the  second  attempt,  made 
by  Darwin,  who  had  at  his  command  such  vast  accumu- 
lations of  well-attested  facts,  was  all  the  more  thorough. 
In  availing  himself  of  recent  progress,  the  latter  was  able 
to  employ  quite  other  scientific  evidence  than  Lamarck  and 
Geoffrey,  Goethe  and  Treviranus,  could  command.     But,  in 
the  second  place,  we  must  give  due  weight  to  the  fact  that 
Darwin  has  the  especial  merit  of  having  approached  the 


DARWIN  S   ARGUMENT.  95 

question  from  an  entirely  new  direction,  and  of  having 
worked  out  that  independent  theory  in  explanation  of  the 
Doctrine  of  Descent  which  we  properly  call  the  Darwinian 
Theory,  or  Darwinism. 

While  Lamarck  explained  the  variation  of  organisms 
descended  from  common  ancestral  forms,  as  especially  the 
effect  of  habit  and  the  use  of  the  organs,  but  also  by  the 
aid  of  the  phenomena  of  Heredity,  Darwin  independently, 
and  on  an  entirely  new  basis,  unfolded  the  actual  causes 
which  mechanically  accomplish  the  modification  of  the 
various  animal  and  vegetable  forms  by  the  aid  of  Adap- 
tation and  Heredity.  Darwin  deduced  his  "  Theory  of 
Selection"  from  the  following  considerations.  He  com- 
pared the  origin  of  the  various  breeds  of  animals  and  plants 
which  man  is  able  to  produce  artificially, — the  conditions 
of  "  Selection "  in  horticulture,  and  in  the  breeding  of 
domestic  animals, — with  the  origin  of  wild  species  of 
plants  and  animals  in  a  natural  state.  He  thus  found 
that  causes  similar  to  those  which,  in  artificially  breeding 
domestic  animals,  and  raising  cultivated  plants,  we  apply 
to  alter  the  forms,  are  also  at  work  in  Nature.  He  named 
the  most  effective  of  all  the  co-operating  causes  the 
Struggle  for  Existence.  The  gist  of  Darwin's  theory, 
properly  so  called,  is  this  simple  idea :  that  the  Struggle 
for  Existence  in  Nature  evolves  new  Species  without  design, 
just  as  the  Will  of  Man  produces  new  Varieties  in  Culti- 
vation with  design.  Just  as  the  gardener  and  the  farmer 
breed  for  their  own  advantage,  and  according  to  their 
own  will,  making  judicious  use  of  the  productive  effects 
of  Heredity  and  Adaptation,  so  does  the  Struggle  for 
Existence  constantly  modify  the  forms  of  vegetables  and 


96  THE   EVOLUTION   OF   MAN. 

animals  in  an  undomesticated  state.  This  Struggle  foi 
Existence,  or  the  universal  efforts  of  organisms  to  secure  the 
necessary  means  of  existence,  works  without  design,  but 
yet  in  the  same  way  modifies  the  organisms.  But  as  under 
its  influence  Heredity  and  Adaptation  enter  into  most 
intimate  reciprocal  relations,  there  necessarily  arise  new 
forms,  or  variations,  which  are  of  advantage  to  the  organ- 
ism, and  which  have,  therefore,  an  object,  although  in 
reality  not  originating  from  a  preconceived  design. 

This  simple  fundamental  idea  is  the  real  gist  of  Darwin- 
ism, or  the  "Theory  of  Selection."  Its  author  conceived 
the  idea  long  ago,  but  with  admirable  industry  he  employed 
twenty  years  in  collecting  data  from  actual  experience  for 
proving  his  theory  before  declaring  it.  In  the  "History 
of  Creation"  (Chapter  VI.),  I  gave  a  full  account  of  the 
method  by  which  he  reached  his  results,  as  well  as  of  his 
most  important  writings,  and  his  life.  I  shall,  therefore, 
now  only  allude  very  briefly  to  some  of  the  most  important 
points.80 

Charles  Darwin  was  born  on  the  12th  of  February,  1809, 
at  Shrewsbury,  where  his  father,  Robert  Darwin,  practised 
as  a  physician.  His  grandfather,  Erasmus  Darwin,  was 
a  thoughtful  naturalist,  who  laboured  in  the  line  of  the 
earlier  natural  philosophy,  and  who,  toward  the  end  of 
the  eighteenth  century,  published  several  works  on  that 
subject.  The  most  important  of  these  is  his  "Zoonomy," 
which  appeared  in  1794,  and  in  which  he  expressed  views 
like  those  of  Goethe  and  Lamarck,  though  he  knew  nothing 
of  the  similar  efforts  of  these  contemporaries.  Erasmus 
Darwin  transmitted  to  his  grandson  Charles,  according  to 
the  law  of  latent  transmission  (Atavism),  certain  mole- 


DARWIN'S  LIFE.  97 

cular  movements  of  the  cells  in  the  ganglia  of  his  powerful 
brain,  which  had  not  made  their  appearance  in  his  son 
Robert.  This  fact  is  of  great  interest  in  relation  to  the 
remarkable  law  of  Atavism  which  Charles  Darwin  himself 
has  so  well  discussed.  But  in  the  writings  of  Erasmus 
Darwin,  formative  imagination  too  greatly  outweighs 
critical  judgment,  while  in  his  grandson,  the  two  are  evenly 
balanced.  As,  at  present,  many  naturalists  of  limited 
genius  regard  imagination  as  superfluous  in  Biology,  and 
their  own  lack  of  it  as  a  great  and  "  exact "  advantage, 
I  take  this  opportunity  of  calling  attention  to  a  striking 
remark  of  an  able  naturalist,  who  was  himself  one  of  the 
leaders  of  the  school  called  "  exact,"  confining  itself  strictly 
to  experience.  Johannes  Miiller,  the  German  Cuvier,  whose 
works  will  always  be  regarded  as  models  of  exact  investiga- 
tion, declared  that  continuous  interaction  and  harmonious 
balance  of  imagination  and  reason,  are  the  indispensable 
conditions  of  the  most  important  discoveries.  This  passage 
is  given  in  full  as  a  motto  at  the  beginning  of  the  eighteenth 
chapter. 

After  completing  his  university  studies  in  his  twenty- 
second  year,  Charles  Darwin  was  fortunate  enough  to 
accompany  an  expedition  which  sailed  round  the  world  for 
scientific  purposes.  This  lasted  for  five  years,  thus  affording 
him  an  abundance  of  most  instructive  suggestions  and  of 
opportunities  for  the  contemplation  of  Nature  in  its 
grandest  forms.  In  the  very  beginning  of  the  voyage, 
when  he  first  landed  in  South  America,  he  noticed  a  variety 
of  phenomena,  which  suggested  to  him  the  great  problem  of 
his  life-long  work,  the  question  of  the  "Origin  of  Species." 
On  the  one  hand,  the  instructive  phenomena  of  the  geogra- 


98  THE   EVOLUTION   OF   MAN. 

phical  distribution  of  species,  and  on  the  other,  the  relation 
between  the  living  and  extinct  species  of  the  same  continent, 
suggested  to  him  the  idea  that  nearly  allied  species  might 
have  descended  from  a  common  ancestral  form.  On  his 
return  from  his  five  years'  voyage,  he  devoted  himself  for 
years  most  zealously  to  the  systematic  study  of  domestic 
animals  and  garden-plants,  and  he  recognized  the  evident 
analogies  between  the  formation  and  transmutations  of  these, 
and  those  of  wild  species  in  a  state  of  nature.  He  had, 
however,  no  conception  of  natural  selection  through  the 
struggle  for  existence,  which  is  the  most  important  feature 
in  the  construction  of  his  theory,  until  after  he  had  read 
the  celebrated  book  of  Malthus,  the  political  economist,  on 
the  "Principles  of  Population."  This  gave  him  a  clear 
conception  of  the  analogy  between  the  changing  relations 
of  population  and  over-population  in  civilized  countries  and 
the  social  relations  of  animals  and  plants  in  a  wild  state. 
He  continued  for  many  years  to  collect  materials  in  order  to 
accumulate  a  great  mass  of  evidence  for  the  support  of  this 
theory.  At  the  same  time,  as  a  practical  breeder,  he  insti- 
tuted many  important  experiments  in  breeding,  and  gave 
special  attention  to  the  instructive  breeding  of  domestic 
pigeons.  Ample  leisure  was.  afforded  him  by  the  quiet 
retirement  in  which,  after  his  return  from  his  journey 
round  the  world,  he  has  lived  on  his  property  of  Down,  near 
Beckenham. 

It  was  not  until  the  year  1858,  that  Darwin  was  induced, 
by  the  work  of  another  naturalist,  Alfred  Russell  Wallace, 
who  had  conceived  the  same  Theory  of  Selection,  to  publish 
the  outlines  of  his  theory.  In  1859  appeared  his  principal 
work,  "  On  the  Origin  of  Species,"  in  which  the  theory  is 


NATURAL   SELECTION.  99 

exhaustively  discussed,  and  is  established  by  the  weightiest 
evidence.  Having  fully  expressed  my  opinion  of  this  book 
in  my  Generelle  Morphologie,  and  in  the  "  History  of 
Creation,"  it  will  here  be  sufficient  to  recapitulate  briefly 
the  gist  of  the  Darwinian  theory,  on  the  right  under- 
standing of  which  everything  depends.  The  whole  is  based 
on  the  simple  fundamental  idea  that  the  Struggle  for 
Existence  in  Nature  modifies  organisms,  and  produces 
new  species  by  the  aid  of  the  same  means  by  which  man 
produces  new  domesticated  varieties  of  animals  and  plants. 
These  means  consist  in  the  constant  preference  or  selection 
of  the  individuals  most  suitable  for  propagation,  so  that  the 
interaction  of  Heredity  and  Adaptation  acts  as  a  modifying 
cause.31 

The  celebrated  traveller  Wallace  had  independently 
formed  the  same  conclusions.  He  had,  however,  by  no 
means  determined  the  influence  of  Natural  Selection  in 
forming  species  as  clearly  and  thoroughly  as  had  been  done 
by  Darwin.  But  the  writings  of  Wallace  (especially  those 
on  Mimicry,  etc.)  contain  many  admirable,  original  con- 
tributions to  the  Theory  of  Selection.  It  is  most  unfor- 
tunate that  the  imagination  of  this  gifted  naturalist  has 
since  become  diseased,  and  that  he  now  only  plays  the  part 
of  a  spiritualist  in  the  spiritualistic  society  of  London. 

The  effect  produced  by  Darwin's  book  on  "  The  Origin  of 
Species  by  Natural  Selection  "  in  the  animal  and  vegetable 
kingdom,  was  extraordinarily  great,  though  not  at  first  in 
the  special  branch  of  science  to  which  it  most  directly 
applied.  Several  years  passed  before  botanists  and  zoolo- 
gists recovered  from  their  surprise  at  the  new  views  of 
nature  advanced  by  this  great  reconstructive  work.  The 


10O  THE   EVOLUTION  OF  MAN. 

effect  of  the  book  on  the  special  sciences  with  which 
zoologists  and  botanists  are  concerned,  has  become  really 
prominent  only  during  the  past  few  years,  during  which  the 
Theory  of  Descent  has  been  applied  in  Anatomy  and  On- 
togeny, and  in  zoological  and  botanical  classification.  In 
some  ways  it  has  already  caused  extraordinary  progress  and 
a  great  reform  in  the  prevailing  views. 

But  in  Darwin's  first  work  of  1859,  the  point  which 
most  interests  us  here — the  application  of  the  Theory 
of  Descent  to  Man — was  not  touched  at  all.  For  many 
years  it  was  even  asserted  that  Darwin  had  no  intention  of 
applying  his  theory  to  Man,  but  that  he  shared  the  preva- 
lent opinion,  that  an  entirely  peculiar  place  in  creation  must 
be  assigned  to  Man.  Not  only  men  unversed  in  science, 
including  very  many  theologians,  but  even  educated  natur- 
alists, asserted  with  the  greatest  ingenuousness,  that  the 
Darwinian  Theory  in  itself  was  not  to  be  combated,  and 
was  entirely  correct,  for  it  afforded  an  excellent  means  of 
explaining  the  origin  of  the  various  species  of  animals  and 
plants;  but  that  the  theory  was  in  no  way  applicable  to 
Man. 

In  the  mean  time,  however,  many  thoughtful  people, 
naturalists  as  well  as  others,  expressed  the  opposite  opinion, 
that  it  necessarily  follows  as  the  logical  conclusion  from  the 
Theory  of  Descent,  as  formulated  by  Darwin,  that  Man 
must  have  descended  from  other  animal  organisms,  and, 
immediately,  from  Mammals  resembling  Apes.  The  truth  of 
this  conclusion  was  early  recognized  by  many  thoughtful 
opponents  of  the  theory.  Just  because  they  regarded  this 
as  a  necessary  consequence,  many  felt  that  the  whole  theory 
must  be  rejected.  The  first  scientific  application  of  this 


HUXLEY   AND   VOGT.  IO1 

theory  to  Man  was  made  by  Huxley,  who  now  holds  the 
first  place  among  English  zoologists.32  This  able  and 
learned  philosopher,  to  whom  much  progress  in  zoological 
science  is  due,  published  a  little  work  entitled  "Evidences 
of  Man's  Place  in  Nature,"  in  the  year  1863,  contain- 
ing three  essays:  1.  On  the  Natural  History  of  Man- 
like Apes  ;  2.  On  the  Relations  of  Man  to  the  Lower 
Animals ;  3.  On  Some  Fossil  Remains  of  Man.  *  In  these 
three  very  important  and  interesting  essays,  it  is  clearly 
shown  that  the  much-disputed  descent  of  Man  from  the  Ape 
is  the  necessary  consequence  of  the  Theory  of  Descent.  If 
the  Theory  of  Descent  is  correct  as  a  whole,  it  is  impos- 
sible not  to  regard  the  Apes  most  resembling  Man  as  the 
animals  from  which  the  human  race  has  been  immediately 
evolved. 

Almost  simultaneously  Karl  Vogt,  a  most  acute  zoologist, 
published  a  larger  work  on  the  same  subject,  entitled 
"Lectures  on  Man,  his  Place  in  Creation  and  in  the  History 
of  the  Earth."  This  author  has  since  partly  retracted  his 
views,  and  has,  indeed,  quite  recently  adopted  the  strange 
assumption  that  the  descent  of  Man  can  only  be  traced 
from  the  Apes,  and  not  from  the  yet  lower  animals.  This, 
however,  only  shows  that  Vogt  has  not  followed  the  recent 
progress  of  Zoology,  and  that  he  has  long  ceased  to  sym- 
pathize with  the  most  important  parts  of  the  History  of 
Evolution. 

Gustav  Jaeger  83  and  Friedrich  Rolle M  must  be  men- 
tioned among  zoologists  who,  after  the  publication  oi 
Darwin's  work,  took  up  the  Theory  of  Descent,  advanced 
it,  and  drew  the  right  logical  conclusion,  that  Man  is 
descended  from  the  lower  animals.  Friedrich  Rolle,  in  1866 


IO2  THE   EVOLUTION   OF  MAN. 

published  a  work  on  "  Man,  his  Descent  and  Civilization,  ir» 
the  light  of  the  Darwinian  Theory." 

At  the  same  time,  in  the  second  volume  of  my  Generelle 
Morphologie  der  Organismen,  which  appeared  in  1866,  I 
made  the  first  attempt  to  apply  the  Theory  of  Evolution  to 
the  entire  classification  of  organisms,  including  Man.35  I 
tried  to  sketch  the  hypothetical  genealogies  of  the  dif- 
ferent classes  of  the  animal  kingdom,  of  the  kingdom  of 
Protista,  and  of  the  vegetable  kingdom,  not  only  as  they 
must  be  according  to  the  principles  of  the  Darwinian 
Theory,  but  also,  as  it  is  already  really  possible  to  do,  with 
a  certain  degree  of  probability.  For,  if  the  Theory  of 
Descent,  as  first  definitely  stated  by  Lamarck,  and  after- 
wards firmly  established  by  Darwin,  is  correct  in  its  general 
principles,  then  it  must  also  be  possible  to  interpret  the 
natural  system  of  plants  and  animals  genealogically,  and  to 
place  the  smaller  and  larger  divisions  recognized  in  the 
system,  as  limbs  and  branches  of  a  genealogical  tree.  The 
eight  genealogical  tables  which  I  appended  to  the  second 
volume  of  the  Generelle  Morphologie,  are  the  first  attempts 
to  accomplish  this.  In  the  twenty-seventh  chapter  of  the 
same  work  are  given  the  most  important  stages  in  the 
ancestral  line  of  the  human  race,  as  far  as  they  can  be 
traced  in  the  descent  of  Vertebrates.  I  there  attempted 
especially  to  determine  the  place  in  the  mammalian  class 
assigned  to  Man  by  the  system,  and,  as  far  as  seems  possible 
at  present,  the  genealogical  significance  of  the  latter.  In 
the  twenty-second  and  twenty-third  chapters  of  my  "  His- 
tory of  Creation,"  I  materially  improved  on  this  attempt 
and  explained  it  in  a  more  popular  form. 

At  lost,  in  1871,  Darwin  himself  published  a  very  in- 


INFLUENCE   OF   SEXUAL   SELECTION.  IO3 

teresting  work,  which  contains  the  much-disputed  applica- 
tion of  his  theory  to  Man,  and  which,  therefore,  completes 
his  great  doctrine.  In  this  work,  entitled  "  The  Descent  of 
Man,  and  Selection  in  Relation  to  Sex,"  s6  Darwin  has 
openly  and  most  logically  drawn  the  inference,  about  which 
he  had  before  purposely  maintained  silence,  that  Man  also 
must  have  been  evolved  from  lower  animals.  In  a  most 
masterly  manner  he  discussed  especially  the  very  important 
part  paid  by  Sexual  Selection  in  the  progressive  exaltation 
of  Man,  and  of  all  other  higher  animals.  According  to  this 
theory,  the  careful  selection  which  the  two  sexes  exercise  on 
each  other,  in  relation  to  their  sexual  connection  and  re- 
production, and  the  aesthetic  taste  evinced  by  the  higher 
animals  in  this  matter,  has  a  most  important  influence  on 
the  progressive  evolution  of  forms  and  in  the  distinction  of 
the  sexes.  The  male  animals  seek  out  the  most  beautiful 
females,  and,  on  the  other  hand,  the  females  choose  the 
finest  males,  so  that  the  specific,  and  at  the  same  time  the 
sexual  character  is  continuously  ennobled.  In  this  respect 
many  of  the  higher  animals  exercise  a  better  taste  and  a 
more  impartial  judgment  than  does  man.  But  even  among 
men  sexual  selection  has  given  rise  to  a  noble  form  of 
family  life,  which  is  the  chief  foundation  on  which  civiliza- 
tion and  social  states  have  been  built.  The  human  race 
certainly  owes  its  origin  in  great  measure  to  the  perfected 
Sexual  Selection  which  our  ancestors  exercised  in  the  choice 
of  wives.  (Cf.  Chapter  XL  of  the  "History  of  Creation," 
and  pp.  24-1-247  in  the  second  volume  of  the  Generelle 
Morphologic.} 

In  all  essential  points  Darwin  approves  of  the  general 
outline  of  the  genealogical  tree  given  in  the  Generelle 


[O4  THE   EVOLUTION   OF  MAN. 

phologie  and  the  "  History  of  Creation,"  and  he  expies&ty 
states  that  his  experience  points  to  the  same  conclusions. 
It  is  impossible  not  to  appreciate  his  great  wisdom  in  not 
himself  applying  the  Theory  of  Descent  to  Man,  in  his  first 
work ;  for  the  inference  was  of  a  sort  to  raise  the  strongest 
prejudices  against  the  entire  doctrine.  It  was  at  first  only 
necessary  to  establish  the  theory  in  relation  to  the  species 
of  animals  and  plants.  Its  application  to  Man  then  inevit- 
ably followed  sooner  or  later. 

It  is  most  important  to  understand  this  connection 
rightly.  If  all  organisms  have  sprung  from  a  common  root, 
Man  is  also  included  in  this  common  descent.  But  if,  on 
the  contrary,  each  separate  kind  or  species  of  organism  has 
been  separately  created,  then  Man  was  also  "  created,  not 
evolved."  Between  these  two  opposite  views  lies  our 
choice;  and  this  decisive  alternative  cannot  be  often 
enough  and  prominently  enough  placed  in  the  foreground. 
Either  all  the  various  species  of  the  vegetable  and  animal 
kingdoms  are  of  supernatural  origin,  created,  not  evolved — 
in  which  case  "Man  is  also  the  product  of  a  supernatural  act 
of  creation,  as  is  assumed  in  all  the  various  systems  of 
religious  belief ;  or,  the  various  species  and  classes  of  the 
animal  and  vegetable  kingdoms  have  evolved  from  a  few 
common  and  most  simple  ancestral  forms ;  and  if  this  is  the 
case,  man  himself  is  the  latest  product  of  the  evolution  of 
the  genealogical  tree  of  animals. 

The  connection  between  the  two  may  be  concisely  stated 
as  follows :  the  Descent  of  Man  from  lower  animals  is  a 
special  deductive  law,  necessarily  following  from  the  general 
inductive  law  of  the  entire  Doctrine  of  Descent.  This 
sentence  formulates  the  relation  most  clearly  and  siraply, 


BIOLOGY  AN  INDUCTIVE   SCIENCE.  IOS 

The  Doctrine  of  Descent  is  really  nothing  but  a  great  in- 
ductive law,  to  which  we  are  led  by  grouping  and  compar- 
ing the  most  important  empirical  laws  of  Morphology  and 
Physiology.  We  are  obliged  to  draw  our  conclusions 
according  to  the  laws  of  induction  in  every  case  in  which 
we  are  unable  to  establish  the  truths  of  nature  immediately 
by  the  infallible  method  of  direct  measurement,  or  mathe- 
matical calculation.  In  the  study  of  animated  nature,  we 
are  seldom  able  entirely  to  ascertain  the  significance  of 
phenomena  immediately,  and  by  infallible  mathematical 
means,  as  is  possible  in  the  much  simpler  study  of  inorganic 
bodies,  in  Chemistry,  Physics,  Mineralogy,  and  Astronomy. 
In  the  last  especially,  we  can  always  employ  the  very 
simple  and  absolutely  sure  method  of  mathematical  calcula- 
tion. But  in  Biology,  this  is  for  many  reasons  entirely 
impossible,  and  especially  because  the  phenomena  in  it  are 
far  too  complex  to  admit  of  immediate  solution  by  mathe- 
matical analysis.  We  are  therefore  compelled  to  proceed 
inductively;  in  other  words,  from  the  mass  of  separate 
observation  we  must  gradually  draw  general  conclusions, 
which  must  be  more  and  more  approximately  correct. 
These  inductive  conclusions,  it  is  true,  cannot  claim  the 
absolute  certainty  of  mathematical  propositions ;  but  they 
are  more  and  more  approximately  true  in  proportion  with 
the  increase  in  extent  of  the  experiences  on  which  they  are 
based.  The  importance  of  such  inductive  laws  is  in  no  way 
lessened  by  the  circumstance  that  they  must  only  be 
regarded  as  provisional  scientific  achievements,  which  may 
possibly  be  improved,  or  perfected,  by  the  further  progress 
of  knowledge.  This  is  equally  true  of  the  greater  part  of 
knowledge  in  other  sciences ;  for  example,  in  Geology  and 


IO6  THE   EVOLUTION   OF   MAN. 

Archaeology.  However  much  particular  items  of  such  induc- 
tive knowledge  may  in  time  be  improved  and  modified, 
their  general  significance,  as  a  whole,  remains  quite  mi- 
touched. 

The  Theory  of  Descent,  according  to  Lamarck  and  Dar- 
win, as  a  great  inductive  law,  and  indeed  the  greatest  of 
all  inductive  biological  laws,  is  in  the  first  place  based  on  the 
facts  of  Palaeontology,  on  the  modification  of  species  brought 
to  light  by  the  science  of  Petrifactions.  From  the  condi- 
tions under  which  these  fossils,  or  petrifactions,  are  found 
buiied  in  the  rock- layers  of  our  earth,  we  draw  the  first 
sure  conclusion,  that  the  organic  population  of  the  earth,  as 
well  as  the  crust  of  the  earth  itself,  has  been  slowly  and 
gradually  evolved,  and  that  series  of  diverse  populations 
have  successively  appeared  at  different  periods  of  the 
earth's  history.  Modern  geology  shows  us  that  the  evolu- 
tion of  the  earth  has  been  gradual,  and  without  total  and 
violent  revolutions.  Comparing  the  various  plant  and 
animal  creations  that  have  successively  appeared  during  the 
course  of  the  earth's  history,  we  find,  in  the  first  place,  that 
an  increase  in  the  number  of  species  has  been  constant  and 
gradual  from  the  earliest  to  the  most  recent  times ;  and,  in 
the  second  place,  we  perceive  that  the  increase  in  the  per- 
fection of  the  forms  belonging  to  each  of  the  larger  groups 
of  animals  and  plants  is  also  constant.  For  example,  the 
only  Vertebrates  existing  in  the  earliest  times  are  the  lower 
Fishes ;  then  the  higher  kinds  of  Fishes ;  later  Amphibia 
appear ;  still  later,  the  three  higher  classes  of  Vertebrates, 
Reptiles  first,  then  Birds,  and  Mammals ;  of  these  only  the 
most  imperfect  and  lowest  forms  appear  first ;  it  is  only  at 
a  very  late  period  that  the  higher  placental  Mammals 


PALAEONTOLOGY   AND   COMPAEATIVE  ANATOMY.         IO/ 

appear,  and  among  the  latest  and  youngest  forms  of  the 
latter  is  Man.  Both  the  perfection  of  forms  and  their 
variety  originate,  therefore,  only  gradually,  and  in  a  period 
extending  from  the  oldest  time  to  the  present  day.  This 
fact  is  of  great  importance,  and  can  be  explained  only  by 
the  Doctrine  of  Descent,  with  which  it  perfectly  agrees.  If 
the  various  groups  of  plants  and  animals  really  descended 
one  from  another,  then  such  an  increase  in  number  and 
degree  of  perfection,  as  the  series  of  fossils  actually  exhibits, 
must  necessarily  have  occurred. 

A  second  series  of  phenomena  of  great  importance  for 
the  inductive  law  with  which  we  are  dealing,  is  contributed 
by  Comparative  Anatomy.  This  latter  is  that  part  of 
Morphology,  or  the  Science  of  Forms,  which  compares  the 
developed  organic  forms,  and  seeks,  in  their  great  variety, 
to  find  the  one  common  law  of  their  organization,  or,  as 
it  was  formerly  called,  the  "general  plan  of  structure."  Since 
Cuvier  first  formed  this  science,  at  the  beginning  of 
this  century,  it  has  always  been  a  favourite  study  of  the 
most  eminent  naturalists.  Goethe,  even  before  him,  had 
been  greatly  attracted  by  the  charm  of  the  mysteries  which 
it  solved,  and  had  been  drawn  into  the  study  of  Morphology. 
It  was  especially  Comparative  Osteology,  the  philosophical 
observation  and  comparison  of  the  bony  skeletons  of  Verte- 
brates, which  is  really  one  of  the  most  interesting  branches 
of  the  science,  that  riveted  his  attention  and  led  him  to  form 
his  Theory  of  the  Skull,  which  has  already  been  mentioned. 
Comparative  Anatomy  teaches  that  in  each  line  of  descent 
in  the  animal  kingdom,  and  in  each  class  in  the  vegetable 
kingdom,  the  inner  structures  of  all  the  animals  belonging 
to  the  one,  and  of  the  plants  belonging  to  the  other,  are  .in 

10 


IO8  THE   EVOLUTION   OF   MAN. 

all  essential  points  in  the  highest  degree  similar,  even 
though  the  outward  forms  are  extremely  unlike.  Man, 
accordingly,  in  all  essential  features  of  internal  organization 
so  closely  resembles  other  Mammals,  that  no  comparative 
anatomist  has  ever  doubted  that  he  belongs  to  that  class. 
The  whole  inner  structure  of  the  human  body, — the  disposi- 
tion of  its  various  systems  of  organs, — the  arrangement  of 
the  bones,  muscles,  blood-vessels,  and  the  like, — the  coarser 
and  more  minute  structure  of  all  these  organs,  corresponds  so 
well  with  that  of  all  other  Mammals, — such  as  Apes,  Gnawing 
animals  (Rodentia),  Hoofed  animals  (Ungulatd),  Whales, 
and  Oppossums, — that  the  complete  dissimilarity  of  the 
outward  form  is  as  nothing  in  the  balance  against  it.  We 
learn  also  from  Comparative  Anatomy  that  the  fundamental 
characteristics  of  animal  organization  are  so  much  alike,, 
even  within  the  various  classes,  numbering  from  thirty 
to  forty  in  all,  that  they  may  fittingly  be  arranged  in  from 
six  to  eight  principal  groups.  But  even  in  these  few  groups, 
which  represent  the  lineages  or  types  of  the  animal  kingdom, 
it  can  be  shown  that  certain  organs,  especially  the  intestinal 
canal,  were  originally  uniform. 

We  can  only  explain  this  most  essential  uniformity  in 
all  these  various  animals,  notwithstanding  their  great  ex- 
ternal dissimilarity,  by  the  aid  of  the  Theory  of  Descent. 
Only  by  considering  the  internal  correspondence  as  the 
result  of  Heredity  from  common  ancestral  forms,  and  the 
external  dissimilarity  as  the  result  of  Adaptation  to  varied 
conditions  of  life,  can  this  wonderful  fact  be  thoroughly 
understood. 

The  recognition  of  this  truth  raised  Comparative 
Anatomy  itself  to  a  higher  rank,  so  that  Gegenbaur,37  the 


"  DYSTELEOLOGY."  1 09 

ablest  living  representative  of  this  science,  could  say  with 
perfect  justice,  that  the  Theory  of  Descent  opened  a  new 
period  in  Comparative  Anatomy,  and  that  the  former  is 
the  touchstone  of  the  latter.  "So  far,  no  experience  in 
Comparative  Anatomy  is  contradictory  to  the  theory  of 
Descent;  all  rather  lead  to  it.  So  that  the  theory  will 
receive  back  from  the  science  that  which  it  has  imparted 
to  its  methods;  namely,  clearness  and  certainty."  Formerly, 
the  remarkable  internal  similarity  of  structure  in  organisms 
had  been  a  source  of  wonder,  incapable  of  explanation. 
Now,  however,  we  can  understand  the  causes  of  these  facts, 
and  can  prove  that  this  wonderful  uniformity  is  simply  the 
necessary  consequence  of  Heredity  from  common  ancestral 
forms,  and  that  the  striking  dissimilarity  of  the  external 
form  is  the  necessary  consequence  of  Adaptation  to  the 
outward  conditions  of  existence. 

There  is  a  special  branch  of  Comparative  Anatomy 
which  is  peculiarly  interesting  in  this  respect,  and  at  the 
same  time  of  the  most  extended  philosophical  significance. 
This  is  the  science  of  Rudimentary  Organs,  which  we  may 
call,  in  reference  to  their  philosophical  consequences,  the 
Doctrine  of  Purposelessness,  or  Dysteleology.  Almost 
every  organism,  with  the  exception  of  the  lowest  and 
most  imperfect,  and  especially  every  highly  developed  vege- 
table or  animal  body,  man  as  well  as  others,  possesses  one  or 
more  structures  which  are  useless  to  its  organism,  valueless 
for  its  life-purposes,  worthless  for  its  functions.  Thus  all  of 
us  have  in  our  bodies  various  muscles  which  we  never  use ; 
for  example,  the  muscles  in  the  external  ear  and  the  parts 
immediately  surrounding  it.  These  outer  and  inner  ear 
muscles  are  of  great  use  to  most  Mammals,  especially  such 


HO  THE   EVOLUTION   OF  MAN. 

as  have  the  power  of  erecting  the  ears,  "because  the  form 
and  position  of  the  ear  may  thus  be  materially  altered,  in 
order  to  take  in  the  various  waves  of  sound  in  the  best 
possible  manner.  In  Man,  however,  and  in  other  animals 
not  possessing  the  power  of  pricking  up  the  ears,  the 
muscles,  though  present,  are  useless.  As  our  ancestors  long 
ago  discontinued  to  make  use  of  them,  we  have  lost  the 
power  of  moving  them.  Again,  there  is  in  the  inner  corner 
of  our  eye  a  small  crescent-shaped  or  semi-lunar  fold  of  skin; 
the  last  remnant  of  a  third  inner  eyelid,  the  so-called  nicti- 
tating membrane.  In  our  primitive  relatives,  the  Sharks, 
and  in  many  other  Vertebrates,  this  membrane  is  highly 
developed,  and  of  great  use  to  the  eye ;  but  with  us  it  is 
abortive  and  entirely  useless.  On  the  intestinal  canal  we 
have  an  appendage,  which  is  not  only  useless,  but  may 
become  very  injurious,  the  so-called  vermiform  appendage 
of  the  caecum.  This  little  appendage  of  the  intestine  not 
infrequently  causes  fatal  disease.  If  in  the  process  of 
digestion,  by  an  unlucky  accident,  a  cherry-stone  or  some 
similar  hard  body  is  pressed  into  its  narrow  passage,  a 
violent  inflammation  ensues,  which  usually  causes  death. 
This  vermiform  appendage  is  not  of  the  slightest  use  in  our 
organism ;  it  is  the  last  and  dangerous  remnant  of  an  organ, 
which  was  much  larger  in  our  vegetarian  ancestors,  and  was 
of  great  use  to  them  in  digestion ;  as  it  is  still  in  many 
herbivorous  animals,  such  as  Apes  and  Rodents,  in  which 
it  is  of  considerable  size,  and  of  great  physiological  im- 
portance. 

Other  similar  rudimentary  organs  exist  in  us,  as  in  all 
higher  animals,  in  different  parts  of  the  body.  They  arei 
among  the  most  interesting  phenomena  with  which  Com- 


RUDIMENTARY   ORGANS.  Ill 

parative  Anatomy  acquaints  us ;  firstly,  because  they  afford 
the  most  obvious  proof  of  the  Theory  of  Descent,  and 
secondly,  because  they  most  forcibly  refute  the  custom- 
ary teleological  philosophy  of  the  schools.  The  Doctrine 
of  Descent  renders  the  explanation  of  these  remarkable 
phenomena  very  simple.  They  must  be  regarded  as  parts 
which  in  the  course  of  many  generations  have  gradually 
been  disused  and  withdrawn  from  active  service.  Owing 
to  disuse  and  consequent  loss  of  function,  the  organs 
gradually  waste  away,  and  finally  entirely  disappear.  The 
existence  of  rudimentary  organs  admits  of  no  other  expla- 
nation. Hence,  they  are  of  the  greatest  philosophical 
importance;  they  clearly  prove  that  the  mechanical,  or 
monistic  conception  of  the  nature  of  organisms  is  alone 
correct,  and  that  the  prevailing  teleological,  or  dualistic 
method  of  accounting  for  them,  is  entirely  false.  The  very 
ancient  fable  of  the  all- wise  plan  according  to  which  "  the 
Creator's  hand  has  ordained  all  things  with  wisdom  and 
understanding,"  the  empty  phrase  about  the  purposive 
"  plan  of  structure  "  of  organisms  is  in  this  way  completely 
disproved.  Stronger  arguments  can  hardly  be  furnished 
against  the  customary  teleology  or  Doctrine  of  Design,  than 
the  fact  that  all  more  highly  developed  organisms  possess 
such  rudimentary  organs. 

The  favourite  phrase,  "  the  moral  ordering  of  the  world," 
is  also  shown  in  its  true  light  by  these  dysteleological 
facts  Thus  viewed,  the  "  moral  ordering  of  the  world  "  is 
evidently  a  beautiful  poem  which  is  proved  to  be  false  by 
the  actual  facts.  None  but  the  idealist  scholar,  who  closes 
his  eyes  to  the  real  truth,  or  the  priest,  who  tries  to 
keep  his  spiritual  flock  in  ecclesiastical  leading-strings,  can 


112  THE   EVOLUTION   OF   MAN. 

any  longer  tell  the  fable  of  "  the  moral  ordering  of  the 
world."  It  exists  neither  in  nature  nor  in  human  life, 
neither  in  natural  history,  nor  in  the  history  of  civilization. 
The  terrible  and  ceaseless  "  Struggle  for  Existence "  gives 
the  real  impulse  to  the  blind  course  of  the  world.  A 
"moral  ordering,"  and  "a  purposive  plan"  of  the  world 
can  only  be  visible,  if  the  prevalence  of  an  immoral  rule 
of  the  strongest  and  undesigned  organization  is  entirely 
ignored. 

The  Natural  System  of  Organisms,  which  classifies  all 
the  various  forms  in  larger  and  smaller  groups,  according  to 
the  degree  of  similarity  or  dissimilarity  of  these  forms,  is 
the  widest  inductive  basis  of  the  Theory  of  Descent. 
These  groups  or  categories  of  the  system,  the  varieties 
species,  genera,  families,  orders,  classes,  and  so  on,  always 
show  such  relative  co-ordination  and  subordination  that 
they  can  be  explained  only  genealogically,  and  the  whole 
system  can  but  be  represented  figuratively  under  the  form  of 
a  tree  with  many  branches.  This  tree  is  the  genealogical 
tree  of  the  groups  related  in  form,  and  their  relation  in 
form  really  is  their  relation  in  blood.  As  no  other  explana- 
tion can  be  given  of  the  fact  that  the  system  naturally 
assumes  a  tree-like  form,  we  may  regard  this  as  an  imme- 
diate and  powerful  proof  of  the  truth  of  the  Doctrine  of 
Descent. 

Among  the  most  important  of  the  phenomena,  testify- 
ing to  the  inductive  law  of  the  Theory  of  Descent,  is  the 
geographical  distribution  of  animal  and  vegetable  species 
over  the  surface  of  the  earth,  and  their  topographical  distri- 
bution on  the  heights  of  mountains  and  in  the  depths  of 
oceans.  Alexander  Humboldt  gave  a  fresh  impulse  to  the 


THEORY    OF   MIGRATIONS.  113 

scientific  investigation  of  these  conditions,  to  the  Science  of 
Distribution,  or  Chorology ;  but  until  Darwin,  people  were 
satisfied  to  observe  the  phenomena  of  Chorology,  and  tried 
principally  to  establish  the  demarcations  of  the  distribu- 
tions of  existing  organic  groups  of  greater  or  less  extent. 
But  the  causes  of  the  remarkable  phenomena  of  distribu- 
tion, the  reasons  why  some  groups  exist  only  here,  others 
only  there,  and  why  there  are  such  numerous  divisions  of 
the  various  species  of  plants  and  animals,  it  was  impossible 
to  explain.  The  Doctrine  of  Descent,  for  the  first  time,  fur- 
nishes the  key  to  the  solution  of  this  problem  also ;  it  alone 
puts  us  in  the  right  way  to  obtain  an  explanation,  by  show- 
ing us  that  the  various  species  and  groups  of  species  spring 
from  common  ancestral  species,  the  widely  diverging  pos- 
terity of  which  gradually  spread  over  the  whole  earth. 
Yet  for  every  group  of  species  there  must  be  assumed  a  so- 
called  "  centre  of  creation  " — that  is,  a  common  cradle,  or 
original  habitat,  in  which  the  common  ancestral  species  of 
a  group  first  evolved,  and  from  which  their  immediate 
descendants  dispersed  in  different  directions.  Individuals 
of  these  migrated  species  became  in  their  turn  the  ances- 
tral species  of  new  groups,  which  again,  by  active  and 
passive  migration,  dispersed;  and  so  on.  As  every  form 
after  its  migration  adapted  itself  to  new  conditions  of 
existence  in  its  new  home,  it  underwent  modification,  and 
gave  rise  to  new  series  of  forms. 

Darwin,  by  the  Theory  of  Descent,  was  the  first  to 
establish  this  highly  important  doctrine  of  active  and 
passive  migrations.  At  the  same  time  he  correctly  pointed 
out  the  significance  of  the  important  chorological  relations 
between  the  living  population  of  each  region  and  their  fossiJ 


£14  THE  EVOLUTION   OF  MAN. 

ancestors  and  allied  forms.  Moritz  Wagner  worked  out  this 
point  most  excellently  under  the  name  of  "  The  Theory  of 
Migration." w  But,  in  our  opinion,  this  famous  traveller 
has  over-estimated  the  importance  of  his  "  Theory  of  Mi- 
gration," in  so  far  as  he  declares  it  to  be  a  condition 
necessary  to  the  rise  of  new  species,  and  holds  the  "  Theory 
of  Selection "  to  be  incorrect.  Tbe  two  theories  are,  how- 
ever, in  no  way  opposed.  On  the  contrary,  migration, 
by  which  the  ancestral  species  of  a  new  kind  becomes 
isolated,  is  only  a  special  form  of  selection.  The  great  and 
interesting  series  of  chorological  phenomena,  since  they  can 
only  be  explained  by  the  Theory  of  Descent,  must  also  be 
considered  as  important  inductive  data  of  the  latter. 

Exactly  the  same  is  true  of  all  the  remarkable  pheno- 
mena which,  in  the  "  Household  of  Nature,"  we  observe  in 
the  economy  of  the  organisms.  All  the  various  relations  of 
animals  and  plants,  to  one  another  and  to  the  outer  world, 
with  which  the  CEkology  of  organisms  has  to  do,  and  espe- 
cially such  interesting  phenomena  as  those  of  parasitism,  of 
family  life,  of  the  care  of  young,  and  of  socialism, — all  admit 
of  simple  and  natural  explanation  only  by  the  Doctrine  of 
Adaptation  and  Heredity.  While  it  was  formerly  usual  to 
marvel  at  the  beneficent  plans  of  an  omniscient  and  bene- 
volent Creator,  exhibited  especially  in  these  phenomena,  we 
now  find  in  them  excellent  support  for  the  Theory  of 
Descent ;  wi.thout  which  they  are,  in  fact,  incomprehensible. 

Finally,  the  whole  of  Ontogeny,  the  history  of  the  indi- 
vidual evolution  of  all  organisms,  is  an  important  inductive 
foundation  of  the  Theory  of  Descent.  But  as  this  subject 
will  be  especially  treated  in  later  chapters,  nothing  further 
iiecd  be  said  of  it  here.  Step  by  step,  I  shall  endeavour 


"  SPECIES."  1 1 5 

to  show  that  the  whole  of  the  phenomena  of  Ontogeny 
forms  a  connected  chain  of  evidence  in  favour  of  the  truth 
of  the  Theory  of  Descent,  and  that  they  can  be  explained 
only  by  Phylogeny.  With  the  aid  of  this  close  causal 
connection  between  Ontogeny  and  Phylogeny,  and  by 
constantly  appealing  to  our  fundamental  law  of  Biogeny, 
we  shall  be  gradually  able  to  prove  from  the  facts  of  On- 
togeny that  Man  is  descended  from  the  lower  animals. 

In  conclusion,  it  must  be  mentioned  that  very  recently 
the  important  theoretical  question  as  to  the  nature  and  idea 
of  "  kind,"  or  "  species,"  which  is  the  point  on  which  really 
hang  all  the  disputes  about  the  Theory  of  Descent,  has  been 
definitely  settled.  For  more  than  a  century  this  question 
was  discussed  from  the  most  varied  points  of  view,  without 
resulting  in  a  satisfactory  settlement.  During  that  time 
thousands  of  zoologists  and  botanists  have  occupied  them- 
selves in  systematically  distinguishing  and  describing 
species,  without,  however,  any  clear  idea  of  the  meaning 
of  "  species."  Many  hundred  thousand  vegetable  and 
animal  forms  were  set  up  and  marked  as  good  species, 
though  even  those  who  declared  them  such  were  unable  to 
justify  the  proceeding,  or  to  give  logical  reasons  for  thus 
distinguishing  them.  Endless  disputes  arose  among  the 
"pure  systematizers,"  on  the  empty  question,  whether  the 
form  called  a  species  was  "  a  good  or  a  bad  species,  a  species 
or  a  variety,  a  sub-species  or  a  group,"  without  the  question 
being  even  put  as  to  what  these  terms  really  contained  and 
comprised.  If  they  had  earnestly  endeavoured  to  gain  a 
clear  conception  of  the  terms,  they  would  long  ago  have 
perceived  that  they  have  no  absolute  meaning,  but  are 
merely  stages  in  the  classification,  or  systematic  categories, 
and  of  relative  importance  only. 


£16  THE   EVOLUTION    OF    MAN. 

It  is  true  that  in  the  year  1857  a  celebrated  and  able, 
but  very  untrustworthy  and  dogmatic  naturalist,  Louis 
Agassiz,  attempted  to  give  an  absolute  signification  to  these 
categories.  He  attempted  this  in  an  "Essay  on  Classification," 
in  which  the  phenomena  of  organic  nature  were  inverted, 
and  in  which,  instead  of  explaining  these  by  natural  causes, 
he  examined  them  through  the  seven-sided  prism  of  theo- 
logical dreams.  Every  "good  species,  or  bona  species"  is, 
according  to  him,  "  an  embodiment  of  a  creative  thought  of 
God."  But  this  fine  phrase  is  as  little  able  to  hold  its 
ground  against  the  criticism  of  natural  science,  as  all  other 
attempts  to  preserve  an  absolute  conception  of  species.  I 
think  I  have  demonstrated  this  sufficiently  in  my  Generelle 
Morphologie  (vol.  ii.  pp.  323-402),  in  the  exhaustive  critique 
there  given  of  the  morphological  and  physiological  idea  of 
species  and  of  systematic  categories. 

Moreover,  Agassiz  can  himself  hardly  have  believed  his 
theosophic  phrases.  This  great  American,  who,  as  Carus 
Sterne  rightly  said,  laid  the  foundation  of  much  natural 
science,39  was,  in  reality,  gifted  with  too  much  genius 
actually  to  believe  in  the  truth  of  the  mystic  nonsense 
which  he  preached.  Crafty  calculation,  and  well-judged 
reliance  on  the  want  of  understanding  of  his  credulous 
followers,  can  alone  have  given  him  courage  to  pass  the 
juggler's  pieces  of  his  anthropomorphic  Creator  as  true  coin. 
The  divine  Creator,  as  represented  by  Agassiz,  is  but  an 
idealized  man,  a  highly  imaginative  architect,  who  is  always 
preparing  new  building  plans  and  elaborating  new  species. 
(Cf.  Chap.  III.  of  the  "  History  of  Creation,"  and  also  "  The 
Aims  and  Methods  of  the  History  of  Evolution."  Jena, 
1875.) 


VARIABILITY   OF   SPECIES.  1 1/ 

When,  in  1873,  the  grave  closed  over  Louis  Agassiz,  the 
last  great  upholder  of  the  constancy  of  species  and  of 
miraculous  creation,  the  dogma  of  the  constancy  of  species 
came  to  an  end,  and  the  contrary  assumption — the  assertion 
that  all  the  various  species  descend  from  common  ancestral 
forms — now  no  longer  encounters  serious  difficulties.  All 
the  elaborate  inquiries  as  to  the  real  nature  of  species,  and 
how  it  is  possible  that  various  species  can  proceed  from 
a  single  ancestral  species,  have  now  been  brought  to  a 
perfectly  satisfactory  close  by  the  fact  that  the  sharp  de- 
marcations between  species  and  variety  on  the  one  side, 
between  species  and  genus  on  the  other,  have  been  entirely 
set  aside.  I  have  given  the  analytical  evidence  of  this  in 
my  "  Monograph  on  Chalk  Sponges,"  *>  which  appeared  in 
1872.  In  it  I  closely  examined  the  variations  of  all  the 
species  of  this  small,  but  highly  instructive  group  of  animals, 
and  demonstrated  in  every  instance  the  impossibility  of 
dogmatic  distinctions  of  species.  Just  in  proportion  as  the 
systematizer  takes  the  ideas  of  Genus,  Species,  and  Varieties 
in  a  wider  or  narrower  sense,  he  distinguishes  in  the  little 
group  of  Chalk  Sponges,  either  only  a  single  genus  with 
3  species,  or  3  genera  with  21  species,  or  21  genera  with 
111  species,  or  39  genera  with  289  species,  or  even  113 
genera  with  591  species.  But  all  these  diverse  forms  are  so 
intimately  connected  by  numerous  transitions  and  inter- 
mediate forms  that  the  common  descent  of  all  the  Chalk 
Sponges  from  a  single  ancestral  form,  the  Olynthus,  can  be 
proved  with  certainty. 

I  think  I  have  thus  given  the  analytical  solution  of  the 
problem  of  the  Origin  of  Species,  and  have  thus  satisfied  the 
demands  of  those  opponents  of  the  Theory  of  Descent  who 


Il8  THE   EVOLUTION    OF   MAN. 

wished  to  see  the  origin  of  allied  species  from  a  single 
ancestral  form  proved  "in  special  instances."  Those  who 
are  not  satisfied  with  the  synthetic  proofs  of  the  truth 
of  the  Doctrine  of  Descent,  as  afforded  by  Comparative 
Anatorr-7  and  Ontogeny,  Palaeontology  and  Dysteleology, 
Chorology  and  Classification,  may  try  to  overthrow  the 
analytic  proofs  .in  the  "  Monograph  on  Chalk .  Sponges," 
which  was  the  product  of  five  years  of  the  closest  observa- 
tion. I  repeat :  if  any  one  still  asserts,  in  opposition  to  the 
Theory  of  Descent,  that  the  derivation  of  all  the  species 
of  a  group  has  hitherto  never  been  convincingly  shown 
in  a  special  instance,  the  assertion  is  now  completely  with- 
out foundation.  The  "  Monograph  on  the  Chalk  Sponges  " 
furnishes  this  analytic  proof  in  detail,  entirely  from  facts, 
and,  as  I  am  convinced,  also  with  incontrovertible  certainty. 
Every  naturalist  who  will  examine  the  extensive  material 
used  in  my  investigations,  and  follow  my  statements,  will 
find  that  in  the  Chalk  Sponges,  the  various  species  can  be 
traced  step  by  step  through  the  course  of  their  evolution  in 
statu  nascenti.  But,  if  this  is  really  the  case,  if,  in  a  single 
class  or  family,  the  derivation  of  all  the  species  from  a 
common  ancestral  form  can  be  shown,  then  the  problem  of 
the  Descent  of  Man  has  been  definitely  solved ;  and  we  are 
able  to  demonstrate  the  derivation  of  man  also  from  lower 
animals. 

The  demand  which  has  been  so  often  made,  and  which 
has  recently  been  repeated  even  by  well-known  naturalists, 
that  the  derivation  of  Man  from  the  lower  animals,  and 
immediately  from  Apes,  yet  requires  "  sure  proof,"  has  thus 
been  satisfied.  These  "sure  proofs"  have  been  for  some 
time  available  to  all  who  would  open  their  eyes  to  see  them 


INSTANCE  OF  THE   ORIGIN   OF  SPECIES.  119 

Quite  vainly,  many  so-called  "  Anthropologists  "  demand  as 
proof,  that  direct  transitional  forms  between  Men  and 
Apes  should  be  found,  or  even  that  a  living  Ape  should 
be  deliberately  cultivated  into  a  Man.  Convincing  and 
"  sure  "  proofs  are  evident  in  the  abundant  material  which 
has  already  been  accumulated.  The  invaluable  sources 
of  Comparative  Anatomy  and  Ontogeny  afford  the  surest 
proof  of  Phylogeny.  It  is,  therefore,  unnecessary  to  search 
out  fresh  proofs  of  the  descent  of  the  human  race,  though 
it  is  necessary  to  recognize  and  to  learn  to  understand  the 
"  sure  proofs  "  which  have  been  obtained. 


CHAPTEE  VI. 
THE  EGG-CELL  AND  THE  AMCEBA. 

The  Egg  of  Man  and  of  otter  Animals  is  a  Simple  Cell.— Import  and 
Essential  Principles  of  the  Cell  Theory. — Protoplasm  (Cell-substance), 
and  the  Nucleus  (Cell-kernel),  as  the  Two  Essential  Constituent  Parts 
of  every  Genuine  Cell. — The  Undifferentiated  Egg-cell,  compared  with  a 
highly  Differentiated  Mind-cell  or  Nerve-cell 'of  the  Brain.— The  Cell  £>s 
an  Elementary  Organism,  or  an  Individual  of  the  First  Order. — The 
Phenomena  of  its  Life. — The  Special  Constitution  of  the  Egg-cell.— 
Yelk.— The  Germ-vesicle. — The  Germ-spot.— The  Egg-membrane,  or 
Chorion.— Application  of  the  Fundamental  Principle  of  Biogeny  to 
the  Egg-cell. — One-celled  organisms. — The  Amoebae. — Organization  and 
Vital  Phenomena. — Their  Movements. — Amoeboid  Cells  in  Many-celled 
Organisms. — Movements  of  such  Cells,  and  Absorption  of  Solid  Matter. — 
Absorbent  Blood  Corpuscles. — Comparison  of  Amoeba  with  Egg-cell. — 
Amoeboid  Egg-cells  of  Sponges. — The  Amoeba  as  the  Common  Ancestral 
Form  of  Many-celled  Organisms. 


"  The  ancestors  of  the  higher  animals  must  be  regarded  as  one-celled 
beings,  similar  to  the  Amoebae  which  at  the  present  day  occur  in  our  rivers, 
pools,  and  lakes.  The  incontrovertible  fact  that  each  human  individual 
develops  from  an  egg,  which,  in  common  with  those  of  all  animals,  is  a 
simple  cell,  most  clearly  proves  that  the  most  remote  ancestors  of  man 
were  primordial  animals  of  this  sort,  of  a  form  equivalent  to  a  simple  cell. 
When,  therefore,  the  theory  of  the  animal  descent  of  man  is  condemned  as 
a  'horrible,  shocking,  and  immoral'  doctrine,  the  unalterable  fact,  which 
caii  be  proved  at  any  moment  under  the  microscope,  that  the  hatnan  egg 


THE   HUMAN    EGG-CELL.  121 

is  ft  simple  cell,  which  is  in  no  way  different  to  those  of  other  mammals, 
must  equally  be  pronounced  '  horrible,  shocking,  and  immoral.'  " — STAMM- 
BAUM  DES  MENSCHENGESCHLECHTS  (1870.) 

IN  order  clearly  to  understand  Ontogeny,  or  the  evolution 
of  the  individual  Man,  the  most  significant  of  the  many 
wonderful  and  varied  facts  which  meet  us  must  first 
be  brought  into  prominence,  and  then  from  the  important 
points  of  view  thus  gained,  the  innumerable  less  weighty 
and  important  phenomena  must  be  explained.  The  first 
and  most  important  point  of  view,  and,  therefore,  the 
starting-point  of  our  ontogenetic  studies,  is  the  fact  that 
every  human  individual  is  developed  from  an  entirely 
simple  cellular  egg.  The  human  egg-cell  is,  in  its  whole 
form  and  constitution,  not  essentially  different  from  those 
of  other  Mammals,  though  there  is  some  difference  between 
the  egg-cells  of  Mammals  and  those  of  other  animals. 

This  most  important  fact,  the  fundamental  significance 
of  which  is  hardly  surpassed  by  any  other,  is  of  recent 
discovery.  It  was  only  in  1827  that  Baer,  by  practical 
observation,  discovered  the  human  and  mammalian  egg. 
Before  that,  the  larger  vesicles,  which  in  reality  contain  the 
true  and  much  smaller  egg,  had  been  erroneously  regarded 
as  the  eggs.  Of  course  the  important  discovery  that  the 
mammalian  egg  is  a  simple  cell  like  that  of  other  animals 
could  only  be  made  after  the  establishment  of  the  Cell 
Theory,  which  was  first  laid  down,  with  respect  to  plants, 
by  Schleiden,  and  extended  to  the  animal  kingdom  by 
Schwann  in  1838.  The  reader  is  already  aware  of  the 
great  importance  of  the  Cell  Theory  in  the  complete  ex- 
planation of  the  human  organism  and  its  evolution.  It 
therefore  seems  desirable  to  say  a  few  words  as  to  the 


122 


THE  EVOLUTION   OF   MAX. 


present  position   of  the  cell  theory,  and    as  to  the  views 
commonly  held  in  connection  with  it. 


FIG.  1.— The  human  egg  from  the  ovary  of  the  female ;  much  enlarged 
The  entire  egg  is  a  simple,  globular  cell.  The  greater  part  of  the  spherical 
egg-cell  is  formed  by  the  egg-yelk,  or  the  granular  cell-snbstance  (proto- 
plasm), which  is  composed  of  innumerable,  delicate  yelk-grannies,  with  a 
little  intervening  substance.  The  germ-vesicle,  answering  to  the  cell- 
kernel  (nucleus)  lies  in  the  upper  part  of  the  yelk.  It  contains  a  dark 
nucleolus'or  germ-spot.  The  globular  mass  of  yelk  is  surrounded  by  a 
thick  transparent  egg-membrane  (zona-  pellucida,  or  clwrion).  This  is. 
penetrated  by  the  pore-canals,  in  the  form  of  very  numerous  hair-like  lines, 
which  run  radially  towards  the  centre  of  the  globe ;  through  these  the 
thread-shaped,  moving  sperm-cells  pass,  in  the  process  of  impregnation,  into 
the  egg-yelk. 

In  order  rightly  to  appreciate  the  Cell  Theory,  which 


THE    CELL    A    UNIT   OF   LIFE.  123 

has  been  regarded  during  the  last  thirty-five  years  as  the 
true  basis  of  all  morphological  and  physiological  know- 
ledge in  Zoology  and  Botany,  it  is  especially  necessary  to 
conceive  the  cell  as  an  integral  organism,  or,  in  other  words, 
an  independent  living  being.  When  by  dissection  we  have 
separated  the  developed  body  of  a  Man,  or  of  any  other 
animal  or  plant,  into  its  organs,  and  when  we  then  proceed 
further  to  examine  by  means  of  the  microscope  the  more 
minute  constituents  of  these  larger  organs,  which  give  the 
form  to  the  whole  organism,  we  are  surprised  to  find  that  all 
these  various  parts  are  made  up  of  the  same  fundamental 
constituents  or  structural  elements ;  and  these  are  cells. 
Whether  we  examine  anatomically  and  by  means  of  the 
microscope,  a  leaf,  a  flower,  or  a  fruit ;  or  again,  a  bone,  a 
muscle,  a  gland,  or  a  piece  of  skin,  etc.,  we  everywhere  find 
one  and  the  same  form-element,  which  has  been  called  the 
Cell,  since  Schleiden  gave  it  that  name.  Very  different 
views  are  held  as  to  the  real  nature  of  this  cell ;  but  what- 
ever we  think  of  it,  it  must  be  regarded  as  an  independent 
life-unit.  The  tiny  cell  is,  as  Briicke  says,  "  an  elementary 
organism,"  or,  as  Virchow  expresses  it,  a  "  seat  of  life " 
(Lebenshcerd}.  It  is,  perhaps,  most  accurately  described  as 
the  organic  unit  of  form  of  the  lowest  grade,  as  an  indi- 
vidual of  the  first  order  (Generelle  Morphologic,  vol.  i. 
p.  269).  This  unit  is  such  both  in  anatomical  form,  and  in 
physiological  function.  In  the  Protista,  in  the  one-celled 
plants  and  primitive  animals,  the  whole  organism  per- 
manently consists  only  of  a  single  cell.  On  the  contrary,  in 
most  animals  and  plants,  it  is  only  in  the  earliest  period 
of  individual  existence  that  the  organism  is  a  simple  cell; 

it  afterwards  forms  a  cell-society,  or,  more   correctly,  an 

11 


124  THE   EVOLUTION    OF   MAN. 

organized  cell-state.  The  human  body  is  not  in  reality  a 
simple  life-unit,  as  is  at  first  the  universally  current,  simple 
belief  of  men.  It  is,  rather,  an  extremely  complex  social 
community  of  innumerable  microscopic  organisms,  a  colony 
or  a  state,  consisting  of  countless  independent  life-units,  of 
different  kinds  of  cells.41 

The  term  cell  is,  in  reality,  not  well  chosen.  Schleiden, 
who  first  introduced  it  as  a  scientific  term  in  the  sense  in 
which  it  is  used  in  the  cell  theory,  named  the  little  element- 
ary organisms  "cells,"  because  in  a  cross-section  of  most 
parts  of  plants,  they  look  like  chambers,  which,  like  the  cells 
of  a  honeycomb,  are  massed  together,  are  separated  by  solid 
walls,  and  are  filled  with  liquid  or  a  soft  pulpy  substance. 
This  conception  of  the  cell,  as  held  by  Schwann,  namely, 
that  it  was  a  small  closed  sac,  or  bladder,  filled  with  a 
fluid,  and  surrounded  by  a  solid  envelope,  or  wall,  continued 
prevalent  for  a  long  time ;  but  in  the  case  of  most  of  the 
cells  in  the  animal  body,  it  is  altogether  inapplicable.  The 
further  the  investigation  of  the  cells  of  the  animal  body  was 
carried,  the  more  evident  it  became  that  the  cell  must  be 


FIG.  2. — Ten  cells  from  the  liver ;  one  (b)  has  two  kernels. 

FIG.  3. — Three  epithelial  cells  from  the  mucous  membrane  of  the  tongue. 


SIMPLE   CELLS.  i2$ 

entirely  differently  conceived.  The  cell  is  now  usually 
defined  as  a  small  semi-solid  or  semi-fluid  (i.e.  neither  solid 
nor  fluid)  dense  body,  the  chemical  nature  of  which  is  albu- 
minous, and  in  which  another  small  roundish  body,  generally 
more  solid,  and  also  albuminous,  is  enclosed.  An  envelope 
or  membrane  may  exist,  as  is  the  case  with  most  plant- 
cells ;  but  it  may  be  wanting,  as  in  most  animal-cells. 
Originally  it  is  never  present.  The  young  cells  are  usually 
roundish  in  form,  but  they  afterwards  vary  very  greatly. 
The  cells  from  different  parts  of  the  human  body,  repre- 
sented in  Figures  2-6,  may  be  compared  as  examples. 


-/•'•;- 


FIG.  4.— Five  thorny,  or  heckle-cells,  the  edges  of  which  fit  into  each 
other,  from  the  epidermis  ;  one  (6)  is  separated  from  the  rest. 

The  most  essential  feature  in  the  modern  conception  of 
the  cells  is,  therefore,  that  the  cell-body  is  composed  of  two 
distinct  parts.  The  one  constituent  part  is  the  inner,  and 
is  called  the  nucleus  (cytoblastus) ;  this  is  generally  of  a 
round,  oval,  or  spherical  form,  usually  more  solid,  seldom 
softer  than  the  protoplasm,  and  consists  of  a  peculiar 
albuminous  substance,  the  nuclein  or  kernel-substance ;  the 
second  essential  constituent  part  of  every  cell  is  the  cell- 
slime  or  cell-substance — the  protoplasm,  or  primitive  slime 
(  Ursckleim  of  the  older  natural  philosophers).  This  proto- 
plasm, which  surrounds  the  nucleus,  also  belongs,  in  point 
of  chemical  composition,  to  the  class  of  albuminous  sub- 
stances, and  is  a  compound  of  carbon,  containing  some 


126 


THE   EVOLUTION   OF   MAN. 


atoms  of  nitrogen.  It  remains  throughout  life  in  a  soft 
condition  of  density,  or  aggregation,  neither  solid  nor  fluid. 
The  albuminous  composition  of  the  protoplasm  is  similar 
to  that  of  the  nucleus,  but  is  yet  essentially  and  constantly 
diverse. 


FIG.  5. — Nine  star-shaped  bone-cells  with  branched  processes. 


FIG.  6. — Eleven  star-shaped  enamel  cells  from  a  tooth  ;  they  are  con- 
nected by  their  branched  processes. 


NEEVE-CELLS.  127 

Nucleus  and  protoplasm,  the  inner  cell-kernel  and  the 
outer  cell-slime,  are  the  only  two  essential  constituents  of 
every  genuine  cell.  Everything  else  which  occurs  in  and 
on  the  cell,  is  of  secondary  importance,  as  it  develops  after- 
wards ;  the  membrane,  which  may  be  variously  constituted, 
fr.nd  is  often  very  thick  ;  the  intermediate  cell-mass,  or  inter- 
cellular substance,  which  is  secreted  between  the  contiguous 
cells ;  and  also  the  bodies  of  various  kinds  contained  in  the 
cell,  such  as  fatty  particles,  crystals,  grains  of  colouring 
matter,  water-vesicles,  etc.  All  these  are  subordinate  and 
passive  parts,  which  are  formed  by  the  activity  of  the 
protoplasm  or  are  taken  up  from  without,  and  are  of  no 
interest  to  us  at  present.  The  nucleus  and  the  protoplasm 
are  the  only  two  active,  essential,  and  always  present  parts 
of  the  cell-organism. 

In  contrast  to  the  simple  cell  (Fig.  1,  p.  122),  let  us 
compare  with  it  a  large  nerve-cell,  or  ganglion-cell  of  the 
brain.  The  egg-cell  potentially  represents  the  whole 
animal — that  is,  it  possesses  the  capacity  to  develop  from 
itself  the  entire  multi-cellular  animal  body;  it  is  the 
common  mother  of  all  the  generations  of  innumerable  cells, 
which  form  the  various  tissues  of  the  animal  body :  in  a 
certain  sense  it  unites  in  itself  their  various  powers,  but 
only  potentially,  only  in  design.  In  direct  contrast  to  this, 
the  nerve-cell  of  the  brain  (Fig.  7)  is  an  extremely  one- 
sided formation.  It  cannot,  like  the  egg-cell,  develop 
from  itself  numerous  generations  of  cells,  of  which  some 
transform  themselves  into  skin-cells,  some  into  flesh-cells, 
and  others  into  bone-cells,  etc.  But  instead,  the  nerve-cell 
which  is  formed  for  the  highest  activities  of  life,  possesses 
the  capacity  to  feel,  to  will,  to  think.  It  is  a  true  mind- 


128 


THE   EVOLUTION   OF  MAN. 


n 


STRUCTURE   OF   NERVE-CELLS.  I2Q 

FIG.  7. — A.  large  branched  nerve-cell,  or  "  mind-cell,"  from  the  brain  of 
au  Electric  Fish  (Torpedo)  ;  600  times  the  natural  size.  The  large,  bright, 
globular  kernel  (nucleus)  lies  in  the  centre  of  the  cell ;  this  nucleus  contains 
a  nucleolus,  and  in  that,  again,  there  is  a  nucleolinus.  The  protoplasm  of 
the  cell  has  separated  into  innumerable  fine  threads  (or  fibrillae),  which  are 
embedded  in  the  inter-cellular  substance,  and  which  pass  out  into  the 
branched  processes  of  the  cell.  An  unbranched  process  (a)  passes  over 
into  a  nerve  vessel.  (After  Max  Schultze.) 

cell,  an  elementary  organ  of  mental  activity.  Correspond- 
ingly, it  has  an  extremely  complex  minute  structure.  Innu- 
merable filaments  of  exceeding  fineness,  which  may  be  com- 
pared to  the  numerous  electric  wires  of  a  great  central 
telegraph  station,  traverse,  crossing  each  other  again  and 
again,  the  finely  granulated  protoplasm  of  the  nerve-cell 
and  pass  into  branched  processes,  which  proceed  from  this 
mind-cell,  and  connect  it  with  other  nerve-cells  and  nerve- 
fibres  (a,  &).  It  is  scarcely  possible  to  trace,  even  approxi- 
mately, the  tangled  paths  of  these  filaments  in  the  fine 
substance  of  the  protoplasmic  body. 

We  thus  have  before  us  a  highly  complex  apparatus, 
the  more  minute  structure  of  which  we  have  hardly  begun 
to  know,  even  with  the  help  of  our  strongest  microscope, 
and  the  significance  of  which  we  rather  guess  than  know. 
Its  complex  mechanism  is  capable  of  the  most  intricate 
psychical  functions.  But  even  this  elementary  organ  of 
mental  activity,  of  which  there  are  thousands  in  our  brain, 
is  only  a  single  cell.  Our  whole  intellectual  life  is  but  the 
sum  of  the  results  of  the  activity  of  all  such  nerve-cells  or 
mind -cells.  In  the  centre  of  each  cell  lies  a  large  trans- 
parent ball,  which  encloses  a  smaller  dark  body.  This  is 
the  nucleus  which  contains  the  nucleolus.  Here,  as  every- 
where, the  nucleus  determines  the  individuality  of  the 
cell,  and  shows  that  the  entire  formation,  notwithstanding 


I3O  THE   EVOLUTION   OF   MAN. 

its  minute  and  complex  structure,  is  in  form  only  a  single 
cell. 

In  contrast  to  this  highly  complex  specialized  mind- 
cell  (Fig.  7)  is  the  egg-cell  (Fig.  1),  which  is  in  no  way 
specialized.  Yet  here,  also,  we  are  obliged  to  infer  from  its 
active  properties  a  highly  complex  chemical  composition  of 
its  protoplasmic  substance,  and  a  minute  molecular  struc- 
ture, which  aie  completely  hidden  from  our  eyes. 

The  description  of  these  cells  as  elementary  organisms, 
or  individuals  of  the  first  order,  must  be  somewhat  qualified. 
For  cells  by  no  means  represent  quite  the  lowest  grade  of 
organic  individuality,  as  that  is  usually  understood.  There 
are  yet  more  simple  elementary  organisms  at  which  we 
will  now  give  a  passing  glance,  in  order  to  return  to 
them  hereafter.  These  are  cytods :  living,  independent 
existences  which  consist  merely  of  an  atom  of  plasson ;  in 
other  words,  of  an  entirely  homogeneous  atom  of  an  albu- 
minous substance,  which  is  not  yet  differentiated  into 
nucleus  and  protoplasm,  but  exercises  the  properties  of  both 
united.  For  example,  the  remarkable  Monera  are  cytods 
of  this  kind.  (Of.  Chapter  XVI.)  Strictly  speaking,  we 
should  say :  the  elementary  organism,  or  the  individual  of 
the  first  order,  occurs  in  two  different  grades.  The  first  and 
lowest  is  the  cytod,  which  consists  merely  of  an  atom  of 
simple  plasson.  The  second  and  higher  grade  is  "the  cell, 
which  has  been  differentiated  into  nucleus  and  protoplasm. 
Both  grades,  cytods  and  cells,  are  grouped  together  under 
the  idea  of  sculptors  or  builders,  because  they  alone  in 
reality  build  the  organism.42  But  in  higher  animals  and 
plants,  such  cytods  do  not,  as  a  rule,  appear,  so  that  only 
actual  nucleated  cells  occur.  -Here,  therefore,  the  elementary 


CYTODS  AND   CELLS.  131 

individual  always  consists  of  two  different  parts,  the  outer 
protoplasm  and  the  inner  nucleus. 

In  order  to  be  thoroughly  convinced  that  every  cell  is 
an  independent  organism,  it  is  only  necessary  to  trace  the 
active  phenomena  and  the  development  of  one  of  these  tiny 
bodies.  We  then  see  that  it  performs  all  the  essential  life- 
functions  which  the  entire  organism  accomplishes.  Every 
one  of  these  little  beings  grows  and  feeds  itself  indepen- 
dently. It  assimilates  juices  from  without,  absorbing  them 
from  the  surrounding  fluid;  the  naked  cells  can  even 
take  up  solid  particles  at  any  point  of  their  surface,  and 
therefore  eat  without  using  any  mouth  or  stomach. 
(Of.  Fig.  15.)  Each  separate  cell  is  also  able  to  re- 
produce itself  and  to  increase  (Fig.  8).  This  increase 
generally  takes  place  by  simple  division,  the  nucleus  parting 
first,  by  a  contraction  round  its  circumference,  into  two 
parts;  after  which  the  protoplasm  likewise  separates  into 
two  divisions.  The  single  cell  is  also  able  to  move  and 


FIG.  8.  —  Blood-cells,  which  increase  by 
division,  from  the  embryo  of  a  young  stag. 
Each  blood-cell  has  originally  a  kernel,  and  is 
globular  (a).  When  they  are  about  to  in- 
crease, the  cell-kernel,  or  nucleus,  first  separ- 
ates into  two  kernels  (b,  c,  d).  The  protoplas- 
mic body  then  becomes  pinched  in  at  a  point 
between  the  two  kernels,  which  become  more 
widely  separated  from  each  other  (e) ,  Finally 
a  complete  separation  between  the  two  parts 
is  effected  at  the  point  where  the  original  cell 
was  pinched  in,  so  that  there  are  now  two 
cells  (/).  (After  Frey.) 

creep  about,  if  it  has  room  for  free  motion,  and  is  not  pre- 
vented by  a  solid  covering ;  from  its  outer  surface,  it  sends 


132 


THE   EVOLUTION   OF   MAX. 


out  and   draws   back   again,  finger-like   processes,  thereby 
modifying  its  form  (Fig.  9).     Finally,  the  young  cell  has 


FIG.  9. — Active  cells  from  the 
inflamed  eye  of  a  Frog  (from  the 
watery  moisture  of  the  eye,  the 
humor  aqueus).  The  naked  cells 
move  freely  and  creep  about; 
like  AmcebsB  and  Rhizopods  they 
accomplish  this  by  extending  deli- 
cate processes  from  their  naked 
protoplasmic  bodies.  These  pro- 
cesses continually  alter  in  number, 
form,  and  size.  The  kernel  of  these 
amoeboid  lymph-cells  is  not  visible, 
being  covered  by  the  numerous  deli- 
cate granules  which  are  scattered 
in  the  protoplasm.  (After  Frey.) 


feeling,  and  is  more  or  less  sensitive.  It  performs  certain 
movements  on  the  application  of  chemical  and  mechanical 
irritants.  Thus  we  can  trace  in  every  single  cell  all  the 
essential  functions,  the  sum  of  which  constitute  the  idea  of 
life :  feeling,  motion,  nutrition,  reproduction.  All  these 
properties  which  the  multi-cellular,  highly  developed  animal 
possesses,  appear  in  each  separate  cell,  at  least  in  its  youth. 
There  is  no  longer  any  doubt  about  this  fact,  and  we  may 
therefore  regard  it  as  the  basis  of  our  physiological  idea  of 
the  elementary  organism. 

Without  lingering  here  over  the  extremely  interest- 
ing phenomena  of  cell-life,  we  will  at  once  attempt  to 
apply  the  Cell  Theory  to  the  egg.  The  comparison  which 
we  have  made  leads  to  the  important  result  that  we 
must  regard  every  egg  as  originally  a  simple  cell.  This 
is  of  the  highest  significance,  because  the  whole  Science  of 


NUCLEUS   AND   NUCLEOLUS.  133 

Ontogeny  can  be  demonstrated  in  answer  to  the  problem : 
"  How  does  a  many-celled  organism  arise  from  a  one-celled 
organism  ? "  Every  individual  organism  is  originally  n 
simple  cell,  and  as  such,  an  elementary  organism,  or  an 
individual  of  the  first  order.  It  is  only  at  a  later  period 
that  this  cell  gives  rise,  by  division,  to  a  multitude  of  cells. 
from  which  the  many-celled  organism,  an  individual  of  a 
higher  order,  is  developed. 

If  we  next  observe  somewhat  more  closely  the  original 
composition  of  the  egg-cell,  we  notice  the  very  remark- 
able fact,  that  in  its  original  condition  it  is  so  exactly 
the  same  in  Man  as  in  all  other  animals,  that  it  is  im- 
possible to  discover  any  essential  difference.  At  a  later 
period,  the  eggs,  even  when  they  remain  one -celled,  are 
very  different  in  size  and  form,  have  different  coverings,  etc. 
But,  if  they  are  sought  in  the  place  where  they  originate, 
in  the  ovary  of  the  female  animal,  these  primitive  eggs,  in 
the  first  stages  of  their  life,  are  found  to  be  always  of  the 
same  formation — every  primitive  egg  being  originally  an 
entirely  simple,  somewhat  round,  moving,  naked  cell,  pos- 
sessing no  membrane,  and  consisting  only  of  the  nucleus 
and  protoplasm  (Fig.  10).  These  two  parts  have  long 
borne  distinctive  names;  the  protoplasm  being  called  the 
vitellus,  or  yelk,  and  the  nucleus  the  germinal  vesicle, 
(vesicula  germinativd).  As  a  rule,  the  nucleus  of  the  egg 
is  of  a  soft,  often  vesicular  texture.  Within  this  nucleus, 
as  in  many  other  cells,  is  enclosed  a  third  body,  which  in 
ordinary  cells  is  called  the  nucleolus.  In  the  egg-cell  it  is 
called  the  germinal  spot  (macula  germinativa).  Lastly,  in 
many,  but  not  in  all  eggs,  within  this  germinal  spot,  is  found 
yet  another  little  point,  a  nucleolinus,  which  may  be  called 


THE   EVOLUTION   OF   MAN. 


FIG.  10. — Primitive  eggs  of  various  animals,  performing  amoeboid  move- 
ments (very  much  enlarged).  All  primitive  eggs  are  naked  cells,  capable  of 
change  of  form.  Within  the  dark,  finely  granulated  protoplasm  (egg-yelk) 
lies  a  large  vesicular  kernel  (the  germ -vesicle),  and  in  the  latter  is  a 
nucleolus  (germ-spot) ;  in  the  nucleolns  a  germ-point  (nucleolinus)  is  often 
visible.  Fig.  A  1 — A  4.  The  primitive  egg  of  a  Chalk  Sponge  (Leuculmis 
echinus),  in  four  consecutive  conditions  of  motion.  Fig.  B  1 — B  8.  The 
primitive  egg  of  a  Hermit-crab  (Chondracanthus  cornutus),  in  eight  conse- 
cutive conditions  of  motion  (after  E.  van  Beneden).  Fig.  C  1 — C  5. 
Primitive  egg  of  a  Cat,  in  four  different  conditions  of  motion  (after  Pfluger). 
Fig.  D.  Primitive  egg  of  a  Trout.  Fig.  E.  Primitive  egg  of  a  Hen.  Fig. 
J<,  Primitive  human  egg. 


EGG-CELLS.  135 

the  germinal  point  (punctum  germinativum).  But  these 
last  two  parts,  the  germinal  spot  and  the  germinal  point,  are 
only  of  subordinate  importance ;  only  the  first  two  parts  are 
of  primary  importance,  the  protoplasm  (Vitellus)  and  the 
nucleus  (vesicula  germinativa). 

In  many  lower  animals,  for  example,  in  Sponges  and 
Medusce,  the  egg-cells  retain  their  entirely  simple  original 
nature  until  fertilization.  But  in  most  animals  they 
undergo  certain  changes  before  that  time ;  they  sometimes 
acquire  certain  additional  Protoplasm,  which  serves  for  the 
nourishment  of  the  egg  (food-yelk),  sometimes  outer  en- 
velopes or  membranes,  which  serve  for  its  protection 
(egg-membranes).  An  envelope  of  this  sort  appears  on  all 
mammalian  eggs  in  the  course  of  their  further  develop- 
ment. The  little  sphere  is  surrounded  by  a  thick  covering 
of  a  perfectly  transparent,  glass-like  nature,  which  is  dis 
tinguished  as  the  zona  pellucida,  or  chorion  (Fig.  11).  When 
this  is  very  closely  examined  under  the  microscope,  very 
fine  radial  lines  may  be  distinguished,  traversing  the  zona ; 
these  are  very  fine  canals.  The  human  egg  cannot  be 
distinguished  from  that  of  most  other  Mammals  either 
in  its  immature  or  in  its  more  complete  condition.  Its 
form,  its  size,  its  composition,  are  approximately  the  same 
in  all.  In  its  fully  developed  condition,  it  has  an  average 
diameter  of  -^  of  a  line,  or  0'2  millimetres.  If  the  mam- 
malian egg  is  properly  isolated  and  held  on  a  glass  plate 
toward  the  light,  it  appears  to  the  naked  eye  as  a  very  fine 
point.  The  eggs  of  most  of  the  higher  Mammals  are  of 
exactly  the  same  size.  The  diameter  of  the  spherical  egg- 
cell  almost  always  measures  from  TL  to  ^y  of  a  line  (£ — ^ 
of  a  millimetre).  It  has  always  the  same  spherical  form, 


THE   EVOLUTION   OF   MAX. 


always  the  same  characteristic  thick  covering ;  always  the 
same  clear,  round  germinal  vesicle  with  its  dark  germinal 
spot.  Even  under  the  highest  magnifying  power  of  the 


FIG.  11. — A  human  egg  (much  enlarged)  from  the  ovary  of  a  female. 
The  whole  egg  is  a  simple  spherical  cell.  The  greater  part  of  this  cell  is 
formed  by  the  egg-yelk,  by  the  granular  cell-substance  (protoplasm),  con- 
sisting of  innumerable  yelk-granules  with  a  little  inter-granular  substance. 
In  the  upper  part  of  the  yelk  lies  the  bright,  globular,  germ-vesicle,  corre- 
sponding with  the  cell-kernel  (nucleus).  This  contains  a  darker  germ-spot, 
answering  to  the  nucleolus.  The  globular  yelk  mass  is  surrounded  by 
a  thick,  light-coloured  egg-membrane  (zona  pellucida,  or  chorion).  This  is 
traversed  by  very  numerous  hair-like  lines,  radiating  towards  the  central 
point  of  the  mass ;  these  are  the  porous  canals,  through  which,  in  the  course 
of  fertilization,  the  thread-shaped,  active  sperm-cells  penetrate  into  the 
egg-yelk. 


IDENTITY   OF   ALL  PRIMITIVE   EGGS.  137 

best  microscope,  there  appears  to  be  no  essential  difference 
between  the  eggs  of  Man,  of  the  Ape,  of  the  Dog,  etc. 
This  does  not  mean  that  they  are  not  really  different 
in  these  different  Mammals.  On  the  contrary,  we  must 
assume  that  such  differences,  at  least  in  point  of  chemical 
composition,  exist  universally.  Even  of  human  eggs,  each 
differs  from  the  other.  In  accordance  with  the  law  of 
individual  variation,  we  must  assume  that  "all  individual 
organisms  are,  from  the  very  beginning  of  their  in- 
dividual existence,  different,  though  often  very  similar." 
(Gen.  Morph.  vol.  ii.  p.  202).  But  with  our  rough  and 
incomplete  apparatus  we  are  not  in  a  position  actually 
to  perceive  these  delicate  individual  differences,  which 
must  often  be  sought  only  in  the  molecular  structure.  Yet 
in  spite  of  this,  the  remarkable  morphological  similarity 
of  human  and  mammalian  eggs,  which  has  the  appearance 
of  absolute  similarity,  remains  a  strong  argument  in  favour 
of  the  common  descent  of  Man  and  the  other  Mammals. 
The  similar  embryo-form  bears  witness  to  the  common 
parent-form.  On  the  other  hand,  there  are  striking  pecu- 
liarities by  which  the  ripe  mammalian  egg  may  be  very 
easily  distinguished  from  the  ripe  eggs  of  Birds  and  other 
Vertebrates.  (Of.  the  end  of  Chapter  XXV.) 

The  ripe  egg  of  the  Bird  is  especially  different,  although 
as  a  primitive  egg  (Fig.  10,  E]  it  was  entirely  similar  to 
that  of  Mammals.  But  the  egg-cell  of  the  Bird  at  a  later 
period,  though  while  still  within  the  oviduct,  absorbs  a  mass 
of  food  which  it  elaborates  into  the  large  and  well-known 
yellow  yelk.  If  a  very  young  egg  from  the  ovary  of  a  he» 
is  examined,  it  is  found  to  be  exactly  like  the  young  egg- 
cells  of  Mammals  and  other  animals  (Fig.  10).  But  it 


138  THE   EVOLUTION   OF   MAN. 

afterwards  grows  so  considerably  that  it  expands  to  the 
well-known  yellow  ball  of  yelk.  The  nucleus,  or  the  germi- 
nal vesicle,  of  the  egg-cell,  is  thus  pressed  on  to  the  surface 
of  the  spherical  cell,  and  is  there  embedded  in  a  small  mass 
of  clear,  so-called  white  yelk.  This  then  forms  a  circular 
white  spot,  which  is  called  the  tread,  or  cicatricle  (cica- 
tricula,  Fig.  12,  6).  From  the  tread  a  thin  cord  of  white 
yelk  passes  through  the  yellow  to  the  middle  of  the  round 
cell,  where  it  swells  to  a  little,  central  ball,  the  falsely-called 
yelk-cavity  (latebra,  Fig.  12,  d}.  The  yellow  yelk,  which 
surrounds  this  white  yelk,  appears  in  the  hardened  egg 
in  concentric  layers  (c).  The  yellow  yelk  is  encircled  by 
a  delicate  structureless  yelk-skin  (membrana  vitellina,  a). 

Of  late  it  has  been  widely  believed  that  the  large  yellow 
egg-cell  of  the  Bird,  which  in  the  case  of  the  largest  birds 
reaches  a  diameter  of  several  inches,  cannot  be  regarded  as 
a  simple  celL  But,  with  Gegenbaur,  we  believe  this  view 
to  be  erroneous.  The  unimpregnated  and  unsegmented  egg- 
cell  of  the  Bird,  with  its  simple  nucleus,  remains  a  simple 
cell,  even  though  its  yellow  yelk-substance  increases  very 
greatly.  Every  animal  which  consists  of  a  single  cell,  every 
Amosba,  every  Gregarina,  every  Infusorial  animal,  is  one- 
celled,  and  remains  so,  however  much  food  of  various  kinds 
it  absorbs.  In  the  same  way  the  egg-cell  remains  a  simple. 
cell,  however  much  food-yelk  it  may  afterwards  collect 
within  its  protoplasm.  Gegenbaur  has  proved  this  clearly 
in  his  excellent  work  on  the  embryos  of  Vertebrates.43 

The  Bird's  egg,  of  course,  assumes  a  different  form  as 
soon  as  it  is  fertilized.  Its  germinal  vesicle,  or  nucleus,  then 
separates  by  repeated  division  into  many  parts,  and  the 
protoplasm  of  the  tread,  which  surrounds  it,  is  corre- 


DEVELOPMENT   OF   THE    EGG-CELL. 


139 


spondingly  divided.  At  this  stage  the  egg  consists  of 
as  many  cells  as  there  are  nuclei  in  the  tread.  Hence,  the 
yellow  ball  of  yelk  of  the  impregnated  egg,  as  it  is  laid, 
and  as  we  eat  it  every  day,  is  already  a  many-celled  body. 
Its  tread  is  composed  of  many  cells,  and  is  now  dis- 
tinguished as  the  germ-disc  (discus  blastodermicus).  In 
the  eighth  chapter  we  shall  refer  to  this  again. 


FIG.  12. — A  ripe  egg-cell  from  the  ovary 
of  a  hen.  The  yellow  nutritive  yelk  (c)  is 
composed  of  many  concentric  strata  (<i)  and 
is  surrounded  by  a  thin  yelk-membrane  (a). 
The  cell-kernel,  or  germ-vesicle,  lies  in  the 
npper  part,  in  the  tread  (6).  From  this  the 
white  yelk  passes  into  the  centre  of  the 
yelk-cavity  (d').  The  two  kinds  of  yelk 
are  not,  however,  distinctly  separated. 


After  the  ripe  egg  of  the  Bird  (Fig.  12)  has  left  the  ovary 
and  has  been  fertilized  in  the  oviduct,  it  surrounds  itself 
with  various  coverings  which  are  secreted  from  the  inner 
surface  of  the  oviduct.  The  thick  layer  of  transparent 
albumen  first  forms  round  the  yellow  yelk  ;  this  is  followed 
by  the  formation  of  the  outer  calcareous  shell,  within  which 
lies  another  envelope  of  skin.  All  these  coverings  and 
additions  which  are  gradually  formed  around  the  egg,  are  of 
no  importance  to  the  development  of  the  embryo  ;  they  are 
parts  that  have  nothing  to  do  with  the  original  simple  egg- 
cell.  Even  in  the  case  of  other  animals  we  often  find  very 
large  eggs  with  thick  coverings, — for  example,  in  that  of 
the  Shark.  In  this  case  also  the  egg  is  originally  exactly 
similar  to  those  of  Mammals,  that  is,  it  is  a  simple  naked 
cell.  But,  as  in  the  case  of  Birds,  a  considerable  quantity 
12 


I4O  THE   EVOLUTION    OF   MAN. 

of  food-yelk  is  collected  within  the  original  yelk,  a3  pro- 
vision for  the  growing  embryo  :  various  coverings  are 
formed  around  the  egg.  The  egg-cells  of  many  other 
animals  receive  similar  internal  and  external  additions. 
But  as  these  are  always  of  subordinate  importance  in  the 
formation  of  the  embryo  itself,  serving  either  as  food,  or  as 
a  protecting  covering  for  the  embryo,  we  may  disregard 
them  entirely,  and  turn  our  attention  to  the  most  important 
point,-^the  essential  similarity  of  the  original  egg-cells  of 
Man  and  other  animals  (Fig  10). 

Let  us  here  make  use  for  the  first  time  of  our  funda- 
mental biogenetic  law,  and  apply  this  causal  law  of  develop- 
ment directly  to  the  human  egg-cell.  This  results  in  an 
extremely  simple,  but  highly  important  conclusion.  From 
the  one-celled  organization  of  the  human  egg  and  of  the 
eggs  of  the  other  animals,  the  conclusion  directly  follows, 
according  to  this  fundamental  law  of  Biogeny,  that  all 
animals,  including  Man,  descend  originally  from  a  one- 
celled  organism.  If  that  fundamental  principle  is  really 
true,  if  germ-history  or  the  development  of  the  individual 
is  an  epitome  or  a  brief  reproduction  of  the  tribal  history  or 
the  development  of  the  race  (and  it  is  impossible  to  doubt 
this),  then,  from  the  fact  that  all  eggs  are  originally  simple 
cells,  we  must  necessarily  conclude,  that  all  many-celled 
organisms  are  descended  from  a  one-celled  organism.  As, 
however,  the  original  egg-cell  has  the  same  structure  in  the 
case  of  Man  as  in  that  of  all  other  animals,  we  may  reason- 
ably assume  that  this  one-celled  original  form  was  probably 
the  common  one-celled  ancestral  organism  of  the  whole 
animal  kingdom,  including  Man.  But  this  last  hypothesis  is 
by  no  means  as  certain  as  the  former  conclusion. 


The  inference  from  the  one-celled  germ-form  to  the 
one-celled  parent-form  is  so  simple,  and  yet  so  full  of  sig- 
nificance, that  it  is  impossible  to  lay  too  much  stress  upon 
it.  Naturally  the  first  question  arising  is,  whether  there 
exist  at  the  present  day  any  one-celled  organisms,  from  the 
form  of  which  we  may  draw  an  approximately  correct 
conclusion  as  to  the  one-celled  ancestors  of  many-celled 
organisms  ?  The  answer  to  this  question  is  undoubtedly 
affirmative.  There  are  most  certainly  one-celled  organ- 
isms now  in  existence,  the  whole  organization  of  which  is 
but  that  of  a  permanent  egg-cell;  there  are  independent 
one-celled  organisms,  which  never  undergo  any  further 
development,  which  pass  their  whole  lives  as  simple  cells, 
and  as  such  reproduce  themselves  without  attaining  to  any 
higher  development.  We  now  know  a  great  number  of  such 
one-celled  organisms, — for  example,  the  Gregarina,  Flagellata, 
Acineta,  Infusoria,  etc.  But  one  among  them  is  especially 
interesting  to  us,  because  it  affords  the  most  complete 
answer  to  our  question,  and  must  be  regarded  as  the  one- 
celled  primary  organism  which  most  nearly  approaches  the 
type-form  of  the  race.  This  organism  is  the  Ainceba. 

The  name  Amcebte  has  long  been  applied  to  a  number  of 
microscopic  one-celled  organisms,  which  are  by  no  means 
rare,  and  are  indeed  widely  distributed,  principally  in  fresh 
water,  but  also  in  the  ocean ;  lately  they  have  been  found 
inhabiting  moist  earth.  When  one  of  these  living  Amcebse 
is  placed  in  a  drop  of  water  under  the  microscope  and 
greatly  magnified,  it  appears  to  be  a  roundish  body  of  very 
irregular  and  varying  form  (Fig.  13).  Enclosed  in  the  soft, 
slimy,  half-fluid  body,  which  consists  of  protoplasm,  we  can 
only  see  a  small  solid  or  vesicular  substance,  which  is  the 


I42  THE   EVOLUTION    OF   MAN. 

nucleus.  This  unicellular  body  now  begins  to  move,  and 
crawls  about  in  various  directions  on  the  glass,  on  which  we 
are  observing  it.  The  shapeless  body  accomplishes  these 

FIG.  13. — A  creeping  Amoeba  (much  en. 
larged).  The  form-value  of  the  whole  or- 
ganism is  that  of  a  simple  naked  cell,  which 
moves  about  by  means  of  variable  processes, 
sometimes  extended  from  the  protoplasm  of 
its  body,  sometimes  drawn  in.  In  the  centre 
is  the  round  kernel,  or  nucleus,  with  its  nn- 
cleolus. 

movements  by  extending  finger-like 
processes  from  various  points  of  its 
surface,  which  are  moved  in  slow  but 
constant  alternations,  and  draw  the  rest  of  the  body  after 
them.  After  a  time  something  new  is  seen ;  the  Amoeba 
suddenly  stands  still,  draws  in  its  processes,  and  assumes 
a  spherical  form.  But  soon  the  little  slimy  ball  begins  to 
spread  out  again,  and  stretches '  its  processes  in  different 
directions,  and  moves  forward  again.  These  variable  pro- 
cesses are  called  false-feet  (Pseudopodia),  because  they 
perform  the  office  of  feet,  and  yet  are  no  special  organs,  in  a 
morphological  sense ;  for  they  vanish  as  quickly  as  they 
appear,  and  are  only  variable  extensions  of  the  semi-fluid, 
homogeneous,  and  structureless  substance  of  the  body. 

If  one  of  these  creeping  Amrebse  is  touched  with  a 
needle,  or  if  a  drop  of  acid  is  added  to  the  water,  the  whole 
body  at  once  contracts  in  consequence  of  this  mechanical  or 
chemical  irritation.  Usually  it  reassumes  its  spherical 
form.  Under  certain  circumstances,  for  example,  if  the 
impurity  is  retained  in  the  water,  the  Amoeba  begins  to 
encase  itself.  It  exudes  a  homogeneous  envelope,  or  cap- 


AMOEBOID    LIFE.  143 

sule,  which  immediately  hardens,  and  in  a  state  of  repose 
assumes  the  form  of  a  spherical  cell  surrounded  by  a  pro- 
tecting membrane.  The  one-celled  Amoeba  obtains  its 
food,  either  by  absorbing  dissolved  substances  directly  from 
the  water  by  imbibition,  or  by  pressing  into  itself  solid 
particles  of  foreign  matter  with  which  it  comes  into  contact. 
The  latter  operation  can  be  observed  at  any  time  if  it  is 
made  to  eat.  If  finely  pulverized  colouring  matter,  such  as 
carmine  or  indigo,  is  placed  in  the  water  in  very  small 
quantities,  the  soft  body  of  the  Amoeba  can  be  seen  to 
assimilate  these  particles  of  colouring  matter,  over  which 
the  soft  substance  of  the  cell  flows  together.  The  Amoeba 
can  take  food  in  this  way  at  any  point  of  the  surface  of  its 
body,  although  it  possesses  no  special  organs  for  taking  in 
and  digesting  nutritive  matter,  no  true  mouth  or  stomach. 
By  means  of  this  assimilation  of  nutriment  and  dissolving 
the  particles  in  its  protoplasm,  the  Amoeba  grows;  and, 
after  it  has  reached  a  certain  size  by  this  process,  it  begins 
to  reproduce.  This  occurs  in  the  simplest  way,  by  division. 
The  enclosed  nucleus  first  separates  into  two  pieces.  Then 
the  protoplasm  distributes  itself  between  the  two  new 
nuclei,  and  the  whole  cell  parts  into  two  similar  cells,  in 
consequence  of  the  growth  of  the  protoplasm  round  the  two 
nuclei.  This  is  the  usual  method  of  propagation ;  the 
nucleus  first  divides  into  two  halves,  which  separate  from 
each  other,  and  act  as  centres  of  attraction  to  the  surround- 
ing cell-substance  or  protoplasm  (Fig.  8). 

Though  the  Amoeba  is,  therefore,  only  a  simple  cell,  it 
shows  itself  capable  of  performing  all  the  functions  of  a 
many-celled  organism.  It  moves  itself  by  creeping,  it  feels, 
it  feeds,  it  reproduces  its  kind.  Some  species  of  Amoebae 


144  THE   EVOLUTION   OF   MAN. 

are  quite  visible  to  the  naked  eye ;  but  the  greater  number 
are  microscopic.  Our  reasons  for  regarding  the  Amoebse  as 
the  particular  one-celled  organisms,  the  phylogenetic  rela- 
tions of  which  to  the  egg-cell  are  of  peculiar  importance, 
will  be  evident  from  the  following  facts.  In  many  lower 
animals,  the  egg-cell  remains  in  its  original,  naked  condition 
till  it  is  fertilized;  it  acquires  no  covering,  and  is  often 
indistinguishable  from  an  Amoeba.  Like  the  latter,  these 
naked  egg- cells  can  extend  processes  and  move  about.  In 
the  Sponges,  these  active  egg-cells  creep  freely  about,  as 
though  they  were  independent  Amoebae  (Fig.  14),  even 

FIG.  14.— Egg-cell  of  a  Chalk  Sponge  (Olyn- 
thus).  The  egg-cell  moves  and  creeps  about  within 
the  Sponge,  by  means  of  variable  processes  which 
it  extends.  It  is  not  distinguishable  from  the 
common  Amoeba. 

within  the  parent  organism.  In  this 
condition  they  were  observed  by  earlier 
naturalists,  and  were  mistaken  for 
Amoebae,  living  as  parasitical  intruders  in  the  body  of 
the  Sponge.  It  was  only  afterwards  that  it  was  dis- 
covered that  these  supposed  one-celled  parasites  were  in 
reality  the  egg-cells  of  the  Sponge  itself.  This  remarkable 
phenomenon  is  also  found  in  other  lower  animals,  for  ex- 
ample, in  those  pretty  bell-shaped  Plant-animals  (Medusce) ; 
the  eggs  of  these  also  remain  as  naked,  uncovered 
cells,  which  stretch  out  amoeboid  processes,  feed  themselves, 
move,  and  from  which,  after  fertilization,  the  many-celled 
Medusa-organism  is  indirectly  or  directly  developed  by 
repeated  division. 

It  is,  therefore,  certainly  no   wild  hypothesis,  but  an 


WHITE   BLOOD-CORPUSCLES. 


145 


entirely  sober  conclusion,  which  regards  the  Amoeba  as  the 
particular  one-celled  organism  which  gives  us  an  approxi- 
mate representation  of  the  ancient  one-celled  ancestral 
form  common  to  all  many-celled  organisms.  The  naked, 
simple  Amoeba  possesses  a  less  differentiated  and  more 
primary  character  than  most  other  cells.  To  this  may  be 
added  the  circumstance,  that  similar  amoeboid  cells  can  be 
shown  in  the  full-grown  bodies  of  all  many-celled  animals. 
For  example,  they  occur  as  the  so-called  white  blood-cor- 
puscles among  the  red  blood-cells  (corpuscles)  in  human 
blood,  and  in  that  of  all  other  Vertebrates.  They  also  occur 
in  many  Invertebrate  animals ;  for  instance,  in  the  blood  of 
the  Snail;  and  in  1859  I  showed  that  these  colourless  blood- 
corpuscles,  like  independent  Amoebae,  can  assimilate  solid 
particles,  can,  therefore,  eat  (Fig.  15).  Lately,  it  has  been 
found  that  very  many  different  cells,  if  they  have  room, 


FIG.  15. — Devouring  blood-cells  of  a  Naked  Sea-snail  (Thetis')  very 
much  magnified.  In  connection  with  the  blood-cells  of  this  snail,  I  was 
the  first  to  observe  the  important  fact  that  "  the  blood-cells  of  invertebrate 
animals  are  uncovered  lumps  of  protoplasm,  and,  like  the  AmcebEe,  by 
means  of  their  peculiar  movements  can  absorb  matter,"  can,  therefore, 
"  eat."  When  at  Naples  (on  the  10th  of  May,  1859)  I  had  injected  the 
blood-vessels  of  one  of  these  Snails  with  pulverized  indigo  dissolved  in 
water,  I  was  much  astonished  to  find  after  a  few  hours  that  the  blood 
cells  themselves  were  more  or  less  filled  with  fine  particles  of  indigo.  By 
repeated  experimental  injections,  I  was  able  to  watch  the  absorption  of  the 
colouring  matter  into  the  blood-cells,  which  was  accomplished  exactly  as  by 
Amoeba?.  (See  "  Monograph  of  Eadiolaria,"  1862,  pp.  104,  105.) 


146  THE   EVOLUTION   OF   MAN. 

are  able  to  move,  to  eat,  and  to  act  entirely  like  Amcebss 
(Fig.  9). 

Tne  capacity  of  the  naked  cell  to  make  these  character- 
istic amoeboid  movements  depends  on  the  contractility  (or 
automatic  movableness)  of  the  protoplasm.  This  seems  to 
be  the  universal  property  of  all  young  cells.  Where  they 
are  not  surrounded  by  a  strong  membrane,  or  shut  up  in  a 
"  cell  prison,"  they  are  all  capable  of  amoeboid  movements. 
This  is  as  true  of  the  uncovered  egg-cell  as  of  other  un- 
covered cells,  of  the  moving  cells  of  various  kinds,  lymph- 
cells,  mucous  cells,  etc. 

Our  examination  of  the  egg-cell  and  comparison  of  it 
with  the  Amoeba,  has  afforded  us  the  best  and  surest  basis 
for  the  history  of  the  germ  as  well  as  for  the  history  of  the 
tribe.  From  it  we  have  drawn  the  conclusions  that  the 
human  egg  is  a  simple  cell ;  that  this  egg-cell  is  not  essen- 
tially different  from  those  of  other  Mammals,  and  that  we 
must  therefore  infer  the  existence  of  a  primeval  one-celled 
ancestral  form,  which  in  all  essential  points  was  of  amoeboid 
form. 

The  assertion  that  the  first  ancestors  of  the  human  race 
were  simple  cells  of  this  sort,  which,  like  the  Amceba,  led 
an  independent  one-celled  life,  has  not  only  been  ridiculed 
as  an  empty  scientific  chimera,  but  has  also  been  indig 
nantly  rejected  in  theological  periodicals  as  "horrible, 
shocking,  and  immoral"  But,  as  I  have  already  remarked 
in  my  lectures  "On  the  Origin  and  Genealogy  of  the  Human 
Race,"  the  same  righteous  indignation  must  fall  with  equal 
justice  on  the  "horrible,  shocking,  and  immoral"  fact,  that 
every  human  individual  develops  from  a  single  cell,  and 
that  this  human  egg-cell  cannot  be  distinguished  from 


UNICELLULAR   HUMAN   GERM.  147 

those  of  other  Mammals.  This  fact  can  be  demonstrated 
at  any  moment  under  the  microscope,  and  it  is  useless  to 
close  our  eyes  to  this  "  immoral "  fact.  It  remains  as 
incontrovertible  as  the  important  conclusions  which  we 
have  linked  with  it. 

The  very  important  bearing  which  the  Cell  Theory  has 
on  the  whole  conception  of  organic  nature  is  thus  very 
clearly  seen.  The  "  place  of  man  in  nature "  is  radically 
explained  by  it.  Without  this  theory,  Man  is  an  unin- 
telligible puzzle.  Philosophers,  therefore,  and  certainly 
psychologists,  ought  especially  to  acquaint  themselves 
thoroughly  with  the  Cell  Theoiy.  The  human  mind  can 
only  be  really  understood  by  means  of  this  theory,  and  its 
simplest  form  is  illustrated  in  the  Amoeba. 

The  extant  Amoebae  and  the  kindred  one-celled  organ- 
isms, Arcellse,  Gregarinse,  etc.,  are  therefore  of  great 
interest,  because  they  show  us  the  simple  cell  in  a  per- 
manently independent  form.  The  human  organism  and 
that  of  other  higher  animals,  on  the  contrary,  is  only  one- 
celled  in  its  earliest,  immature  condition.  As  soon  as  the 
egg-cell  is  fertilized,  it  multiplies  by  division  and  forms  a 
community,  or  colony  of  many  social  cells.  These  dif- 
ferentiate themselves,  and  by  their  specialization,  by  various 
modifications  of  these  cells,  the  various  tissues  which  com- 
pose the  various  organs  are  developed.  The  developed 
many-celled  organisms  of  Man  and  of  all  higher  animals 
resembles,  therefore,  a  social,  civil  community,  the  numerous 
single  individuals  of  which  are/indeed,  developed  in  various 
ways,  but  were  oiiginally  only  simple  cells  of  one  common 
struct  UTQ. 


CHAPTER  YIL 

THE   PROCESSES  OF  EVOLUTION  AND  IMPREGNATION. 

Development  of  the  Many-celled  from  the  -One-celled  Organism. — Tho  Cell- 
hermit  and  the  Cell-state.— The  Principles  of  the  Formation  of  the 
State.— The  Differentiation  of  the  Individuals  as  the  Standard  of  Measure- 
ment for  the  Grade  of  the  State.— Parallel  between  the  Processes  of 
Individual  and  of  Race  Development. — The  Functions  of  Evolution. — 
Growth. — Inorganic  and  Organic  Growth. — Simple  and  Complex  Growth. 
— Nourishment  and  Change  of  Substance. — Adaptation  and  Modification. 
— Reproduction. — Asexual  and  Sexual  Reproduction. — Heredity. — Divi- 
sion of  Labour,  or  Differentiation. — Atavism,  or  Reversion. — Coalescence. 
—The  Functions  of  Evolution  as  yet  very  little  studied  by  Physiology, 
and  hence  the  Evolutionary  Process  has  often  been  misjudged. — The 
Evolution  of  Consciousness,  and  the  Limits  to  the  Knowledge  of  Nature. 
— Fitful  and  Gradual  Evolution. — Fertilization.— Sexual  Generation. — 
The  Egg-cell  and  the  Sperm-cell. — Theory  of  the  Sperm-animals. — 
Sperm-cells  a  form  of  Whip-cell. — Union  of  the  Male  Sperm-cell  with 
The.  Female  Egg-cell.— The  Product  of  this  is  the  Parent-cell,  or 
Cytnla. — Nature  of  the  Process  of  Fertilization. — Relation  of  the  Kernel 
(Nucleus)  to  this  Process. — Disappearance  of  the  Germ-vesicle. — Mone- 
rula. — Reversion  to  the  Monera-form. — The  Cytula. 

"  If  the  man  of  science  chose  to  follow  the  example  of  historians  and 
pulpit-orators,  and  to  obscure  strange  and  peculiar  phenomena  by  employing 
a  hollow  pomp  of  big  and  sounding  words,  this  would  be  his  opportunity  ; 
for  we  have  approached  one  of  the  greatest  of  the  mysteries  which  surround 
the  problem  of  animated  nature  and  distinguish  it  above  all  other  problems 
of  science.  To  discover  the  relations  of  man  and  woman  to  the  egg-cell 
would  be  almost  equivalent  to  solving  all  those  mysteries.  The  origin  and 
development  of  the  egg-cell  in  the  body  of  the  mother,  the  transfer  to  it 


THE  MULTICELLULAK   ORGANISM.  149 

by  means  of  the  seed,  of  the  physical  and  mental  characteristics  of  the 
father,  affect  all  the  questions  which  the  human  mind  has  ever  raised  in 
regard  to  existence." — HUDOLPH  VIKCHOW  (1848). 

THE  discovery  that  every  human  being  at  the  beginning 
of  his  existence  is  a  simple  cell,  that  this  egg-cell  is  essen- 
tially similar  to  those  of  other  Mammals,  and  that  the 
forms  arising  during  the  evolution  of  this  cell  in  Man  and 
in  the  other  higher  Mammals,  are  at  first  similar, — supplies 
a  basis  from  which  we  may  trace  the  further  processes  of 
evolution.  In  the  first  place  we  have  convinced  ourselves 
of  a  fact  which  is  of  great  importance  to  the  empiric  side 
of  the  history  of  development,  relating  to  those  ontogenetic 
facts  which  can  be  directly  traced  by  means  of  the  micro- 
scope;  and  this  fact  is  that  in  Man  as  well  as  in  other 
animals  the  developed  many-celled  organism  with  all  its 
various  organs  proceeds  from  a  simple  cell.  Secondly,  as 
regards  the  phylogenetic  side  of  the  question,  the  specu- 
lative part  of  the  History  of  Human  Development,  which 
is  based  on  those  facts,  we  have  reached  the  conclusion 
that  the  original  ancestral  form  of  Man  as  of  the  other 
animals  was  a  one-celled  organism.  The  whole  difficult 
problem  of  the  History  of  Evolution  is  thus  now  reduced 
to  the  simple  question  :  "  How  has  the  complex  many-celled 
organism  arisen  from  the  simple  one-celled  form  ?  By  what 
natural  process  has  the  simple  cell  been  transformed  into 
that  complex  life-apparatus  with  all  its  various  organs,  the 
apparently  rational  and  purposive  construction  of  which  we 
admire  in  the  developed  body  ?  " 

Turning  now  to  answer  this  question,  we  must  bear  in 
mind  the  view  to  which  we  have  already  alluded,  that  the 
many-celled  organism  is  ordered  and  constituted  on  the 


I5O  THE   EVOLUTION   OF  MAN. 

same  principles  as  a  civilized  state,  in  which  the  several 
citizens  have  devoted  themselves  to  various  services  directed 
towards  common  ends.  This  comparison  is  of  the  greatest 
service  in  enabling  us  thoroughly  to  understand  the  con- 
struction of  Man  from  many  cells  of  various  kinds,  and  to 
understand  also  the  harmonious  co-operation  of  these 
various  cells  for  an  apparently  pre-conceived  purpose.  If 
we  bear  this  comparison  in  mind,  and  apply  this  significant 
idea  of  the  developed  many-celled  organism  as  a  civil  union 
of  many  individuals,  to  the  history  of  the  evolution  of  this 
organism,  we  shall  obtain  a  correct  view  of  the  real  nature 
of  the  first  and  most  important  processes  of  evolution.  We 
can  even,  on  deeper  reflection,  guess  the  first  stages  of 
development,  and  establish  them  cb  priori,  before  we  call 
observation,  d  posteriori  knowledge,  to  our  aid. 

For  once  we  will  reverse  the  process,  and  will  not,  as 
will  be  the  case  hereafter,  first  observe  the  facts  of  Ontogeny 
and  then  attach  their  phylogenetical  significance  to  them. 
Beginning  at  the  other  end,  let  us  here  try  to  guess  the 
course  which  evolution  must  have  taken,  if  the  comparison 
is  well  founded.  Then  if,  afterwards,  the  facts  of  Ontogeny 
confirm  our  preconception,  we  shall  be  yet  more  firmly 
convinced  of  the  truth  of  our  views  on  Phylogeny.  This 
agreement  will  afford  us  a  more  striking  justification  of  our 
views  than  can  be  gained  in  almost  any  other  way. 

Let  us  therefore  first  answer  this  question :  "  Granting 
the  correctness  of  the  fundamental  law  of  Biogeny,  how 
would  the  original  one-celled  organism  which  founded  the 
first  cell-state,  and  thus  became  the  ancestor  of  the  higher, 
many  celled  animals, — how  must  that  organism  have  acted 
at  the  beginning  of  organic  life  on  the  earth,  or  at  the 


THE   CELL   COMMUNITY.  151 

beginning  of  creation,  as  it  is  usually  expressed  ? "  The 
answer  is  very  simple.  It  must  have  acted  just  as  a  man 
who  founds  a  state  or  a  colony  for  a  given  purpose.  Let 
us  trace  this  process  in  its  simplest  form,  as,  for  example, 
may  have  easily  taken  place  when  any  of  the  remote 
islands  in  the  Pacific  Ocean  were  first  peopled.  Two  South 
Sea  Islanders,  a  man  and  a  woman,  have  gone  in  a  boat 
to  fish ;  they  are  overtaken  by  a  storm,  carried  far  away, 
and  at  length  driven  on  to  a  remote  island,  as  yet  unin- 
habited. This  "  first  human  pair,"  remaining  isolated,  play 
the  parts  of  Adam  and  Eve,  and  produce  a  numerous  pos- 
terity, thus  becoming  the  parents  of  the  future  inhabitants 
of  the  island.  As  they  are  entirely  devoid  of  all  resources, 
without  the  many  means  of  support  possessed  by  the 
founders  of  states  of  advanced  civilization,  the  posterity 
of  this  uncivilized  and  isolated  pair  have  first  developed 
as  genuine  savages.  Their  only  purpose  in  life  for  cen- 
turies has  remained  as  simple  as  that  of  the  lower  animals 
and  plants ;  the  simple  aim  of  self-preservation  and  of  the 
production  of  descendants ;  they  have  been  contented  with 
the  simplest  organic  functions,  nutrition  and  reproduction. 
Hunger  and  love  are  their  only  motives  of  action, 

For  a  very  long  period,  these  savages,  scattered  over  the 
whole  island,  must  have  aimed  at  the  one  single  object 
of  self-preservation.  Gradually,  however,  several  families 
collected  at  certain  places,  larger  communities  arose,  and 
now  many  reciprocal  relations  began  to  arise  between 
individuals  ;  in  consequence,  a  rude  division  of  labour  took 
place.  Certain  savages  continued  to  fish  and  hunt,  others 
began  to  cultivate  the  ground,  others  devoted  themselves 
bo  religion  and  medicine,  which  now  began  to  develop, 


152  THE   EVOLUTION   OF   MAN. 

and  so  on.  In  short,  the  ever-increasing  division  of  labcm 
specializes  the  people  into  various  ranks  or  castes,  which 
always  tend  to  become  more  sharply  defined  in  propor- 
tion as  the  state  becomes  more  highly  developed:  all 
follow  diverse  occupations,  and  yet  work  for  a  common  end. 
In  this  way,  from  the  descendants  of  a  single  human  pair, 
a  simple  community  of  individuals,  originally  alike,  first 
gradually  arises,  and  this  is  followed  by  a  more  or  less 
well-organized  confederation.  In  this  community,  we  may 
regard  the  more  or  less  complete  division  of  labour  among 
individuals,  or  the  so-called  specialization,  as  the  standard 
by  which  the  grade  of  development  of  its  culture  may  be 
measured. 

A  process  similar  to  this,  and  the  details  of  which  each 
can  easily  fill  up  for  himself,  took  place  millions  of  years 
ago,  when,  at  the  beginning  of  organic  life  on  the  earth, 
one-celled  organisms  at  first  developed,  and  were  afterwards 
followed  by  many-celled  forms. 

The  single  cells  which  arose  by  reproduction  from  the 
oldest  parent-cells  must  at  first  have  lived  in  an  isolated 
condition;  each  one  performed  the  same  simple  offices  as 
all  the  others;  they  were  satisfied  with  self-preservation, 
nutrition,  and  reproduction.  At  a  later  period  isolated 
cells  gathered  into  communities.  Groups  of  simple  cells, 
which  had  arisen  by  the  continued  division  of  a  single 
cell,  remained  together,  and  now  began  gradually  to  perform 
different  offices  in  life.  The  first  traces  of  specialization,  or 
division  of  labour,  soon  occurred,  as  one  cell  assumed  one 
office,  another  another.  One  set  of  cells  may  have  devoted 
themselves  especially  to  the  absorption  of  food,  or  nutrition; 
other  cells  may  have  busied  themselves  only  with  repro- 


SPECIALIZATION   OF   CELLS.  153 

duetion ;  and  others,  again,  have  formed  themselves  into 
protecting  organs  for  the  little  community,  and  so  on.  In 
short,  various  classes  or  castes  must  have  arisen  in  the 
cell-state,  following  diverse  occupations  and  yet  working 
together  for  the  common  end.  In  proportion  as  this 
division  of  labour  progressed,  the  many-celled  organism, 
or  the  specialized  cell-community,  became  more  perfect  or 
civilized. 

We  may  follow  the  comparison  further.  It  may  be 
asserted  a  priori,  tha't  in  consequence  of  the  reciprocity  of 
relations  which  was  occasioned  by  the  struggle  for  existence 
and  the  gathering  of  many  organic  individuals  in  a  common 
dwelling-place,  when  organic  life  first  began  on  the  earth, 
a  community  of  many  similar  individuals  arose  from  a  one- 
celled  organism ;  that  a  division  of  labour  afterwards  took 
place  among  these  similar  cells,  and  that  finally,  in  conse- 
quence of  continuous  specialization,  a  developed  many- 
celled  organism  with  many  different  organs,  all  working 
for  a  common  end,  arose.  In  order  fully  to  realize  the  value 
of  this  significant  comparison,  it  would  be  necessary  to 
enter  in  detail  into  the  theory  of  the  division  of  labour,  or 
specialization,  which  now  plays  a  very  important  part  in 
Biology,  especially  since  Darwin's  Theory  of  Selection  has 
enabled  us  to  understand  the  true  causes  of  these  phe- 
nomena. At  present  I  must  refer  for  the  more  detailed 
elaboration,  which  would  carry  us  too  far  to  be  entered 
into  here,  to  Darwin's  Doctrine  of  the  Divergence  of  Cha- 
racter, and  to  my  lecture  on  the  Division  of  Labour.  We 
shall  hereafter  return  to  this  subject.44 

At  present  we  will  rather  examine  whether  the  d  priori 
views  on  Phylogeny  which  we  preconceived,  are  in  accord- 


I$4  THE   EVOLUTION   OF   MAN. 

ance  with  the  facts  which  Ontogeny  places  before  us ; 
whether  in  the  evolution  of  the  individual  organism 
from  the  egg-cell,  the  same  phenomena  appear,  which  we 
have  presupposed  as  necessary  in  this  comparison.  The 
ontogenetic  structural  process  proves  to  be  in  very  close 
harmony  with  our  conclusions,  and  we  find  that  the  facts  of 
the  evolution  of  the  individual  which  can  be  seen  under  the 
microscope,  do  in  fact  correspond  perfectly  with  the  picture 
of  the  process  of  phylogenetic  evolution  which  we  have 
sketched  d  priori.  The  first  processes  which  occur  in  the 
evolution  of  the  individual  from  the  egg-cell,  and  also  the 
succeeding  simple  processes  which  first  come  under  observa- 
tion, really  correspond  to  the  events  which  we  have  just 
traced  in  the  development  of  a  colony  of  savages,  and  have 
assumed  as  the  first  phylogenetic  processes  in  the  origin  oi 
a  many-celled  organism. 

In  the  first  stage  of  the  evolution  of  the  individual, 
many  homogeneous  cells  first  arise,  from  the  simple  egg-cell, 
by  continuous  division.  These  are  exactly  comparable  to 
a  community  of  human  beings  as  yet  uncivilized.  These 
homogeneous  cells  increase  still  more,  so  that  the  accu- 
mulation of  cells  ever  increases.  As  in  making  our 
comparison  we  found  that  an  entire  colony  of  savages  pro- 
ceeded from  the  descendants  of  a  single  isolated  human 
pair,  so  likewise  all  the  homogeneous  cells  of  this  multitude 
(which  we  shall  hereafter  learn  to  know  better  under  the 
name  of  cleavage-globules),  are  inter-related  as  the  de- 
scendants of  a  single  pair  of  cells.  Their  common  father 
is  the  male  sperm-cell,  and  their  common  mother  the  female 
egg-cell 

At  first,  all  these  numerous  cells  which  arise  bv  the  con 


LIFE-PHENOMENA.  155 

binuous  division  of  the  fertilized  egg-cell,  are  exactly  alike, 
and  cannot  be  distinguished  from  each  other.  B.ut  gra- 
dually a  division  of  labour  occurs  among  them  by  their 
assuming  different  offices.  Some  accomplish  nutrition,  others 
reproduction,  others  protection,  others  locomotion,  and  so 
on.  We  may  translate  this  into  the  language  of  the  theory 
of  the  tissues  and  say  :  some  of  these  cells  become  intestinal 
cells,  others  muscle-cells,  others,  again,  bone-cells,  nerve-cells, 
cells  of  the  sense-organs,  of  the  reproductive  organs,  etc. 
Thus  we  see  that  the  .whole  course  of  the  evolution  of  the 
individual  corresponds  in  its  essential  features  to  that  pre- 
supposed course  of  phylogenetic  development,  and  thus 
affords  a  striking  confirmation  of  our  fundamental  law  of 
Biogeny. 

This  observation  naturally  leads  to  a  brief  examination 
of  the  physiological  functions,  or  vital  activities,  which  are 
concerned  in  the  evolution  of  the  individual  as  in  that  of 
the  race.  At  first  sight  a  great  number  of  complex  pro- 
cesses seem  to  blend  and  co-operate  here ;  all  of  these  can, 
however,  in  reality  be  reduced  to  a  few  simple  organic 
functions.  These  vital  activities  are :  (1)  Growth ;  (2) 
Nutrition ;  (3)  Adaptation ;  (4)  Reproduction ;  (5)  Heredity  ; 
(6)  Division  of  Labour,  or  Specialization ;  (7)  Atavism ; 
(8)  Coalescence.  Heredity,  Adaptation,  and  Growth  are  of 
especial  importance  in  the  evolution  of  the  organic  body ; 
these  must,  therefore,  be  regarded  as  especially  formative 
functions. 

Of  all  vital  phenomena,  growth  may  be  regarded  as  the 

one  which   plays   the  chief  part  in  the   evolution  of  the 

individual  organism,  and  as  the  really  fundamental  function 

of  evolution.     The  bearing  of  this  function  on  the  evolu- 

13 


156  THE  EVOLUTION   OF  MAN. 

tion  of  the  germ  is  so  great,  that  Baer  expressed  the  most, 
general  result  of  his  researches  in  the  following  proposition : 
"  The  history  of  the  evolution  of  an  individual  is  the  his- 
tory of  the  growth  of  individuality  in  every  relation." 
Whenever  a  unit,  an  individual,  develops  in  nature,  growth 
is  the  first  condition.  This  is  equally  true  of  inorganic 
(inanimate)  and  of  organic  (animate)  natural  bodies.  In 
the  former,  in  minerals,  growth  is  often  the  only  function 
of  evolution.  Growth  is,  therefore,  especially  interesting, 
because  both  in  the  inorganic  individual,  the  crystal,  and 
in  the  simplest  organic  individual,  it  is  the  necessary  pre- 
liminary to  all  further  evolution.  Growth,  the  addition 
of  homogeneous  body-substance,  is  absolutely  universal 
The  inorganic  crystal  grows  by  absorbing  homogeneous 
matter  from  the  surrounding  fluid  medium,  which  then 
passes  from  a  fluid  into  a  solid  condition.  Similarly,  the 
cell,  the  simplest  organic  individual,  grows  by  attracting  to 
itself  particles  from  the  surrounding  medium,  which  is 
usually  fluid,  and  by  then  transforming  these  particles  into 
a  semi-fluid,  and  more  or  less  homogeneous  condition 
(assimilation).  The  only  difference  between  the  growth  of 
the  crystal,  and  that  of  the  simplest  organic  individual,  the 
cell,  is  that  the  former  adds  the  new  substance  externally, 
while  the  latter  absorbs  it  internally.  This  essential  differ- 
ence depends  on  the  different  conditions  of  density,  or  of 
aggregation,  of  the  two  different  groups  of  bodies.  The 
inorganic  bodies  may  be  either  in  a  solid,  fluid,  or  gaseous 
condition.  They  grow  by  apposition.  Organic  bodies,  on 
the  contrary,  are  in  the  fourth,  the  soft  or  semi-fluid  con- 
dition of  aggregation.  They  grow  by  intussusception. 

Each  individual  or  trophic  growth  is,  however,  only  the 


GROWTH.  157 

simple  or  direct  form  of  growth  common  to  crystal  and  to 
simple  organic  individuals  of  the  first  order.  This  simple 
form  of  growth  is  secondarily  opposed  to  compound  or 
numerical  growth,  which  is  seen  in  the  course  of  the  evolu- 
tion of  all  many-celled  organisms,  in  all  individuals  of  the 
second,  or  higher  order.  In  this  case,  the  simple  cell  does 
not  continually  increase,  as  might  be  supposed,  until  the 
whole  large  organic  individual,  with  all  its  parts,  is  formed; 
but  after  the  cell  has  attained  a  certain,  very  limited  size, 
it  does  not  increase  further,  but  parts  by  self-division  into 
two  cells.  Owing  to  the  frequent  repetition  of  this  pro- 
cess of  compound  growth,  a  many-celled  organism,  which 
is  far  larger  than  the  largest  cell,  at  last  arises.  In  this 
case,  the  growth  of  the  ever-increasing  organism  is  no 
longer  the  mere  addition  of  homogeneous  parts,  but  depends 
really  on  generation,  i.e.,  the  multiplication  of  the  origin- 
ally simple  individual. 

A  further  distinction  between  organic  and  inorganic 
growth  depends  on  the  fact  that  the  former,  unlike  the 
latter,  is  connected  with  nutrition.  Nutrition  is  necessary 
to  the  existence  of  every  living  organism,  for  loss  of  sub- 
stance of  body -material  is  implied  in  all  life-energies ;  and 
this  loss  of  substance  must  be  replaced  by  the  addition  of 
new  substance  or  food.  This  continual  change  of  sub- 
stance, the  absorption  and  assimilation  of  new  matter, 
the  expulsion  of  used-up  particles,  and  briefly,  all  the 
processes  included  by  the  term  nutrition,  are  conditions 
as  necessaxy  to  the  accomplishment  of  evolution  as  for  all 
the  other  activities  of  life :  they  are  as  indispensable  to  the 
evolution  of  the  single  cell  as  to  that  of  the  entire  many- 
celled  organism.  The  usual  method  of  nutrition  in  the 


158  THE  EVOLUTION   OF  MAN. 

case  of  the  single  cells  is  by  the  absorption  by  their  soft 
semi-fluid  cell-substance  of  food-material  from  the  sur- 
rounding fluid ;  less  frequently  solid  particles  are  pressed 
into  the  cell-substance.  Similarly,  the  worn-out  material 
is  discharged,  usually  in  a  fluid,  seldom  in  a  solid  form. 

Adaptation,  the  most  important  vital  function,  is 
directly  connected  with  nutrition,  and  plays  the  most  im- 
portant part  in  the  progressive  development  of  the  organism. 
It  is,  in  reality,  the  most  influential  cause  of  every  advance 
and  of  all  perfection  of  the  organism.  Adaptation  effects 
all  the  modifications  or  variations  which  organic  forms 
undergo  under  the  influence  of  the  external  conditions  of 
existence ;  it  is  the  true  cause  of  every  modification.  As 
I  have  very  fully  discussed  the  importance  of  modification 
and  the  various  laws  of  Adaptation  in  my  Generelle 
Morphologic,  and  in  the  "  History  of  Creation,"  I  may  here 
dispense  with  any  further  reference  to  it.  I  shall  only  call 
attention  to  the  fact,  that  all  these  various  laws  of  Adapta- 
tion can  appropriately  be  brought  into  the  two  classes  that 
I  have  there  distinguished ;  on  the  one  side  indirect,  or 
Potential  Adaptation,  on  the  other  direct,  or  Actual  Adap- 
tation. I  have  shown  in  my  Generelle  Morphologic  (vol. 
ii.  pp.  193-226),  that  all  these  varied  and  important  phe- 
nomena, if  regarded  from  a  physiological  point  of  view,  can 
be  reduced  to  the  mechanical  function  of  nutrition,  and, 
indeed,  to  the  elementary  conditions  of  cell-nutrition. 

Just  as  progressive  Adaptation  is  linked  with  nutrition, 
so  is  conservative  Heredity  linked  with  reproduction.  This 
latter  activity  of  the  organism  may  also  be  referred  to  the 
former  functions.  For  radically  "  reproduction  is  a  form  of 
nutrition  and  a  growth  of  the  organism  to  a  size  beyond 


NUTRITION   AND   REPRODUCTION.  159 

that  belonging  to  it  as  an  individual,  so  that  a  part  is  thus 
elevated  into  a  (new)  whole "  (Generelle  Morphologic,  vol. 
ii.  p.  16).  The  functions  of  growth  and  reproduction  are 
therefore  very  intimately  connected.  Reproduction  is  only 
a  continuation  of  the  growth  of  the  individual.  But  the 
latter,  again,  depends  in  its  compound  form,  on  generation, 
that  is,  on  the  multiplication  of  the  simple  constituent  indi- 
viduals. While,  on  the  one  hand,  reproduction  appears  to 
be  only  a  growth  of  the  individual  to  a  size  exceeding  that 
of  the  individual, — compound  growth,  on  the  other  hand, 
is  the  result  of  the  reproduction  of  simple  individuals  of 
the  first  order.  This  view  enables  us  clearly  to  understand 
reproduction  and,  consequently,  Heredity,  which  otherwise 
appears  to  be  an  obscure  and  mysterious  process. 

To  prove  the  correctness  of  this  view,  we  must  start 
from  the  simplest  form  of  reproduction,  that  is,  division,  as 
it  occurs  in  the  case  of  almost  every  cell.  When  the  cell, 


FIG.  16. — Blood-cells  (corpuscles),  increas- 
ing by  self -division,  from  the  blood  of  the  young 
embryo  of  a  stag.  Each  has  originally  a  kernel 
(nucleus),  and  is  globular  (a).  When  the  cells 
are  about  to  multiply,  the  kernel  first  separates 
into  two  (b,  c,  d).  The  protoplasmic  body  then 
becomes  pinched  in  between  the  two  kernels, 
which  separate  more  and  more  from  each 
other  (e).  Finally  the  cell  parts  into  two,  at 
the  point  where  it  was  pinched  in  (/).  (After 
Frey.) 


having,  by  the  absorption  of  nutrition,  already  reached  its 
usual  size,  exceeds  that  measure,  it  divides  into  two  cells 
(Fig.  16).  Just  in  the  same  way  in  many-celled  animals  (for 
example,  Corals),  when  the  individual  grows  beyond  the 


l6o  THE   EVOLUTION   OF   MAN. 

definite  size  proper  to  it,  a  separation  into  two  new 
individuals  necessarily  takes  place.  Starting  from  this 
simplest  form  of  reproduction,  we  can  learn  to  understand 
the  many  complex  forms  with  which  we  meet,  especially  in 
the  lower  animals  and  plants.  Division  is  first  followed  by 
propagation  by  buds,  then  that  by  the  formation  of  germ- 
buds,  and  propagation  by  germ-cells,  or  spores.  All  these 
forms  of  multiplication  are  classed  under  the  name  of 
asexual  reproduction,  or  Monogeny ;  in  these  cases  it  does 
not  require  the  union  of  different  individuals  to  effect  the 
production  of  new,  independent  individuals.45 

The  conditions  of  sexual  reproduction,  or  Amphigony, 
are  quite  different.  Its  nature  consists  in  this ;  that  two 
distinct  cells  must  unite  in  a  particular  way  and  blend  in 
order  to  cause  the  production  of  a  new  individual.  As  we 
shall  soon  return  to  the  subject  of  sexual  reproduction, 
when  we  consider  the  fertilization  of  the  egg,  we  need  not 
here  linger  over  it.  We  must  only  emphasize  the  fact,  that 
this  process  of  sexual  reproduction,  in  spite  of  its  peculiarity, 
is  yet  nearly  related  to  the  higher  forms  of  asexual  repro- 
duction, and  especially  to  that  by  the  formation  of  germ- 
cells.  But  while  in  the  latter  case  a  single  cell  separates 
from  the  confederacy  of  the  many-celled  organism  and 
forms  the  foundation  of  a  new  individual, — in  the  former, 
two  different  elementary  individuals,  a  female  egg-cell  and 
a  male  sperm-cell,  must  unite  and  blend  into  a  single  body 
to  effect  that  purpose.  The  double  cell  formed  in  this  way 
is  alone  capable  of  forming  by  division  an  aggregate  of  cells, 
from  which  a  new  many-celled  organism  then  develops.40 
(Of.  Chap.  XXV.) 

Immediately   connected    with    reproduction   is   a   fifth 


HEREDITY  AND  ADAPTATION.  l6l 

highly  important  evolutionary  function,  Heredity.  Just 
as  we  were  able  to  trace  Adaptation  back  to  nutrition,  we 
can  also  show  that  Heredity  is  a  necessary  phenomenon 
of  reproduction ;  and  this  is  equally  true  of  both  kinds 
of  Heredity — of  conservative,  as  well  as  of  progressive 
Heredity.  As  I  have  also  fully  explained  these  highly 
important  Laws  of  Heredity,  which  maintain  constant 
reciprocal  relations  with  the  Laws  of  Adaptation,  in  my 
"  History  of  Creation,"  vol.  i.  Chapter  VIII.  p.  175,  we 
will  not  stop  to  examine  them  here.  (See  also  Generelle 
Morpholoyie,  vol.  ii.  pp.  170-191.) 

Division  of  labour,  or  differentiation,  which  has  but 
recently  begun  to  be  correctly  valued,  forms  a  sixth 
evolutionary  function  of  especial  importance.  We  have 
already  seen  that  division  of  labour  is  the  strongest  impulse 
towards  progressive  evolution,  not  only  in  civic  and  social 
life,  but  also  in  the  social  cell-confederacy  of  every  many- 
celled  organism.  A  glance  at  any  community  or  state 
organization  shows  that  the  first  condition  of  all  higher 
development  and  civilization,  is,  on  the  one  hand,  the  divi- 
sion of  the  various  duties  among  the  various  classes  of  the 
citizens ;  and,  on  the  other  hand,  the  co-operation  of  these 
single  individuals  for  the  common  purposes  of  the  state. 
This  is  exactly  the  case  also  in  every  many-celled  organism. 
Every  multicellular  individual  in  the  plant  or  animal 
kingdom  is  more  perfectly  developed,  and  ranks  higher  in 
proportion  as  the  division  of  labour  among  its  constituent 
cells,  the  differentiation  of  ;ts  cell-individuals,  is  more 
perfect.  Therefore  in  the  various  classes  of  organisms  we 
find  this  differentiation,  sometimes  in  a  more,  sometimes  in 
a  less  perfect  condition.  The  simplest  form  of  division  of 


1 62  THE   EVOLUTION   OF  MAN. 

labour  occurs  in  those  lower  animals  in  the  bodies  of  which 
only  two  kinds  of  cells  have  become  differentiated.  This 
is  the  case,  for  example,  in  the  lowest  Plant-animals,  in 
Sponges,  and  the  simplest  Polyps,  as  well  as  in  their 
common  parent-form,  the  Gastrsea.  Throughout  the  entire 
many-celled  bodies  of  these,  there  are  only  two  different 
kinds  of  cells  ;  the  one  kind  effect  the  nutrition  and  repro- 
duction of  the  animal,  the  other  kind  are  its  organs  of 
feeling  and  motion.  These  two  kinds  of  cells  are  identical 
with  those  which  first  come  to  perfection  in  the  first  process 
of  differentiation  of  the  germ-layers  in  the  human  embryo. 
But  in  most  higher  animals  the  differentiation  of  the  ceils 
proceeds  much  further.  Some  take  merely  the  office  of 
nutrition ;  others  that  of  reproduction ;  a  third  group  con- 
stitute the  outward  covering  of  the  body  and  form  the 
skin ;  a  fourth  group,  the  muscle-cells,  form  the  flesh ;  a 
fifth  group,  the  nerve-cells,  develop  into  the  organs  of 
sensation,  of  will,  of  thought,  etc.  All  these  different  kinds 
of  cells  originally  proceeded  by  differentiation  or  specializa- 
tion from  the  simple  egg-cell,  and  from  the  homogeneous 
descendants  of  that  egg-cell,  owing  to  division  of  labour. 
This  differentiation  of  the  cells,  or  this  division  of  labour, 
originally  arose  in  tribal  history,  from  causes  similar  to  the 
division  of  labour  in  the  civilized  states  of  men.  Afterwards 
it  appears  in  the  germ-history,  and  by  that  time  it  has  been 
made  over  to  Heredity,  and  is  merely  repeated  in  accord- 
ance with  the  fundamental  law  of  Biogeny.  Now,  although 
Differentiation  usually  leads  to  the  progress  of  the  whole 
organism  as  well  as  of  its  various  constituent  individuals, 
the  single  cells,  yet  it  is  also  in  many  cases  the  occasion  of 
retrogression,  or  atavism.  Not  only  progressive,  but  also 


ATAVISM.  163 

retrograde    modifications    take    place    in    consequence    of 
division  of  labour. 

Atavism,  or  reversion,  must  be  regarded  as  a  seventh 
function  of  evolution,  and,  as  such,  plays  no  unimportant 
part.  In  the  evolution  of  almost  every  higher  organism  we 
observe  that  the  progressive  completion  of  most  organs  is 
accompanied  by  retrograde  processes  of  evolution  in  single 
parts.  In  the  cell  this  retrograde  metamorphosis  usually 
first  occurs  in  consequence  of  the  formation  of  fat-particles 
in  the  protoplasm.  The  cell  is  destroyed  by  the  fatty 
degeneration  of  the  protoplasm.  During  the  course  of 
phylogenetic,  as  of  ontogenetic  evolution,  whole  organs  may 
thus  retrograde  by  the  dissolution  of  the  cells  which  form 
them.  Thus,  for  example,  during  the  evolution  of  the  germ 
of  Man  and  of  other  Mammals,  cartilages,  muscles,  etc.,  dis- 
appear which  were  of  great  importance  in  our  primitive 
ancestors,  the  Fishes.  This  ontogenetic  reversion  reproduces, 
owing  to  Heredity,  a  corresponding  phylogenetic  process. 
The  very  interesting  "  rudimentary  organs  "  are  arrested — 
bodily  growths  of  this  kind,  traces  of  which  still  remain  in 
various  stages  of  development  (see  p.  110).  They  are  found 
in  nearly  every  higher  many-celled  organism  attaining  to 
any  considerable  stag*  of  evolution;  in  this  case  the  general 
progress  of  the  whole  is  scarcely  ever  conditional  on  the 
equally  progressive  development  of  the  cells ;  on  the  con- 
trary, certain  cells  perish  during  Ontogeny,  while  others 
go  on  growing  at  their  expense.  This  same  phenomenon 
is  met  with  in  human  society.  In  this  it  is  always  the 
ca.se  that  many  individuals  perish  without  effecting  any- 
thing; while  the  majority  constantly  develop  more  or 
less  steadily.  The  comparison  is  perfectly  apt.  For  the 


164  THE   EVOLUTION   OF  MAN. 

conditions  of  aggregation  are  the  same  in  states  as  in 
many-celled  organisms. 

Finally,  we  must  mention  an  eighth  and  last  function 
of  organic  development,  viz.  coalescence,  or  concrescence. 
As  yet,  this  has  been  but  little  noticed,  nor  is  it  very 
striking;  yet  it  is  of  real  importance  in  certain  processes. 
Coalescence  consists  in  this,  that  two  or  more  individuals 
which  were  originally  separate  afterwards  combine  a,nd 
blend  into  one  individual.  We  may  regard  the  process 
of  sexual  generation  as  a  coalescence  of  two  cells.  We  also 
often  find  a  similar  coalescence  of  cells  in  other  processes  of 
evolution.  Those  tissues  of  the  animal  body  which  dis- 
charge the  highest  functions,  viz.  the  muscular  tissue,  or 
flesh,  which  is  concerned  in  locomotion,  and  the  nervous 
tissue  which  performs  the  functions  of  sensation,  will,  and 
thought,  consist  in  great  part  of  coalescent  cells.  But  not 
only  cells,  or  individuals  of  the  first  order,  but  also 
organs,  or  individuals  of  the  second  order,  coalesce  very 
freely  in  the  process  of  Ontogeny  into  a  compound 
formation.  Even  independent  organisms  may  coalesce,  as 
is  very  often  the  case,  e.g.  in  the  Sponges.  The  process 
of  coalescence  (often  also  called  conjugation  or  copulation), 
is  in  a  certain  sense  the  opposite  process  to  that  of  propaga- 
tion. In  the  latter  two  or  more  new  individuals  arise 
from  one,  while  in  the  former  one  individual  results  from 
several.  As  a  general  rule,  this  individual  possesses  a  higher 
function  than  that  of  the  two  units  from  the  coalescence  of 
which  it  sprang. 

In  reviewing  for  a  moment  the  different  vital  activities 
of  the  organism  which  we  have  here  enumerated  as  the 
ebsential  functions  of  evolution—  as  the  true  formative  foroea 


INACTIVITY   OF   PHYSIOLOGY.  165 

of  the  nascent  organism — it  will  easily  be  seen  that  they 
all  admit  of  purely  physiological  investigation.  And  yet 
till  very  recently  many  of  them  were  never  closely  studied, 
and  consequently  the  processes  of  evolution  have  very  often 
been  regarded  as  something  altogether  enigmatical  and 
peculiar,  and  even  in  some  respects  miraculous  and  super- 
natural. So  that  even  yet  many  distinguished  naturalists 
hold  that  the  phenomena  of  evolution  are  beyond  the  limits 
of  human  knowledge,  and  are  only  explicable  by  the  as- 
sumption of  supernatural  forces. 

This  curious  situation,  reflecting  as  it  does  a  somewhat 
unpleasant  light  upon  the  present  status  of  our  science, 
must  be  laid  to  the  charge  of  modern  Physiology.  As  I 
have  already  had  occasion  to  remark,  the  Physiology  of  our 
day  pays  no  attention  either  to  the  functions  of  evolution 
or  to  the  evolution  of  the  functions.  With  praiseworthy 
energy  it  has,  it  is  true,  exerted  itself  to  perfect  as  far  as 
possible  the  knowledge  of  certain  groups  of  functions,  to 
which  an  exact  mathematical  and  physical  treatment  is 
directly  applicable  (e.g.  the  Physiology  of  the  sense-organs, 
of  muscular  movement,  of  the  circulation  of  the  blood,  etc.). 
But,  on  the  other  hand,  it  has  paid  but  little  attention  to 
many  important  groups  of  functions,  to  which  this  exact 
method  is  not  applicable.  Among  the  latter  are  the  choro- 
logical  and  cecological  functions,  many  psychological  pheno- 
mena and  correlations  of  growth,  and  especially  the  most 
important  of  those  functions  of  evolution  which  we  have  just 
enumerated — that  of  Heredity  and  Adaptation.  Our  present 
knowledge  of  these  two  most  influential  physiological 
functions  of  evolution  has  been  almost  entirely  acquired 
by  means  of  morphological,  not  physiological  research, 


1 66  THE   EVOLUTION   OF   MAN. 

though  Physiology  had  in  the  pursuit  of  its  own  objects 
occasion  enough  to  devote  itself  earnestly  to  the  study  of 
these  functions.  In  the  same  way  the  important  functions 
of  growth  and  coalescence,  as  also  those  of  differentiation 
and  atavism,  have  as  yet  been  very  little  studied  from  a 
physiological  point  of  view. 

This  neglect  of  the  history  of  evolution  explains  the 
little  interest  and  the  lack  of  insight  exhibited  by  the 
physiologists  of  our  time  with  regard  to  the  theory  of 
descent.  When  Darwin,  in  his  Theory  of  Natural  Selection, 
gave  a  new  basis  to  the  theory  of  evolution,  and  so  pointed 
out  the  way  to  a  physiological  explanation  of  the  formation 
of  species,  a  new  and  most  interesting  field  of  research  was 
thrown  open  to  Physiology.  But  Physiology  has  hardly  yet 
entered  this;  and  it  has  done  as  little  to  advance  our 
knowledge  of  the  processes  of  evolution  in  their  ontogenetie 
as  in  their  phylogenetic  aspect.  In  fact,  with  a  few 
illustrious  exceptions,  most  physiologists  have  paid  very 
little  attention  to  the  theory  of  descent,  and  to  this  day 
some  of  their  most  renowned  leaders  look  on  this  most 
important  biological  theory  as  "an  unproved  and  baseless 
hypothesis." 

This  want  of  comprehension  of  the  history  and  signifi- 
cance of  evolution  can  alone  explain,  for  instance,  the  fact 
that  the  famous  Berlin  physiologist,  Du  Bois-Reymond,  in 
his  well-known  address  "  On  the  limits  of  Natural  Science," 
delivered  at  Leipsic  in  1872,  before  the  meeting  of  German 
naturalists,  declared  human  consciousness  to  be  a  phenome- 
non absolutely  and  unconditionally  transcending  the  bounds 
of  human  comprehension.  It  never  occurred  to  him  that 
consciousness,  in  common  with  every  other  cerebral  activity, 


DU   BOIS-REYMOND.  1 67 

is  ii;  actual  process  of  evolution.  He  overlooked  the  obvious 
consideration  that  even  the  consciousness  of  the  human  race 
must  have  arisen  gradually  by  evolution  through  many 
phylogenetic  stages  precisely  in  the  same  way  that  even  yet 
the  individual  consciousness  of  every  child  is  gradually 
completed  in  the  course  of  many  ontogenetic  stages. 

Again,  this  same  want  of  insight  into  the  functions  and 
the  physiological  process  of  evolution  accounts  for  the  fact 
that  even  at  the  present  day  esteemed  and  learned  natural- 
ists are  earnestly  discussing  the  question  whether  the 
creation  of  species,  or,  in  other  words,  the  phyletic  evolution 
of  forms,  took  place  suddenly  or  gradually.  This  dispute 
is  as  irrational  as  would  be  a  dispute  as  to  whether  the 
mouse  is  a  great  ot  a  small  animal.  The  elephant  will  of 
course  declare  the  mouse  to  be  a  tiny  creature,  while  the 
louse,  living  on  the  skin  of  the  mouse,  must  regard  the 
latter  as  an  animal  of  gigantic  size.  Just  as  in  the  one  case 
the  estimate  of  extension  in  space  is  purely  relative,  and  only 
to  be  taken  in  a  relative  sense,  so  in  the  other  case  is  the 
estimate  of  extension  in  time. 

Every  process  of  evolution  as  such  is  always  continuous, 
and  real  leaps  or  interruptions  never  occur.  Natura  non 
facit  saltus — nature  never  leaps.  This  is  true  both  of  on- 
togenetic and  of  phylogenetic  processes:  of  the  evolution 
of  the  individual  as  well  as  of  that  of  the  species.  It  is 
true  that  in  Ontogeny  leaps  sometimes  appear  to  occur, 
e.g.  when  the  butterfly  is  developed  from  the  pupa  into 
which  the  caterpillar  has  been  transformed,  or  when  a 
Medusa  is  developed  from  an  entirely  dissimilar  hydra-form 
Polyp.  But  the  morphologist  who  step  by  step  studies  the 
exact  course  of  these  processes  of  evolution,  finds  that, 


l6S  THE   EVOLUTION    OF   MAN. 

though  certain  stages  seem  omitted,  the  continuity  is  really 
unbroken,  and  that  each  new  form  arises  directly  from  that 
which  preceded  it.  Throughout  there  is  a  causal  and  un- 
broken connection ;  nowhere  a  sudden  leap.47  But  when  the 
rapidity  of  the  process  of  evolution  is  at  one  time  retarded 
and  again  suddenly  accelerated,  or  when  heredity  is  cur- 
tailed, the  result  of  the  process  appears  to  be  a  sudden  leap. 

This  unbroken  causal  connection  of  the  processes  of 
evolution  exists  equally  in  germ-history,  and  in  tribal 
history.  For  as  Ontogeny  is  but  a  brief  reproduction  of 
Phylogeny,  conditional  on  Heredity .  and  modified  by 
Adaptation,  in  the  latter,  therefore,  as  in  the  former,  no  leap 
or  open  gap  can  ever  really  exist  between  two  consecutive 
evolutionary  forms.  As  in  the  evolution  of  the  individual 
so  in  that  of  the  species,  each  new  form  arises  directly  from 
that  which  preceded  it ;  and  here  also  the  physiological 
process  of  development  always  preserves  its  continuity. 
Even  in  those  extreme  cases  where  a  new  form  does  indeed 
seem  to  come  into  existence  quite  suddenly,  as  in  what  is 
called  "  sudden  or  monstrous  adaptation,"  there  is  always, 
under  the  surface,  an  unbroken  physiological  evolutionary 
process  which  has  the  appearance  of  being  a  "  sudden  leap  " 
only  because  of  its  comparative  rapidity,  or  of  the  magnitude 
of  its  result. 

As  a  striking  instance,  let  us  consider  a  frequently  ob- 
served case  of  such  "sudden  variation."  A  common  two- 
horned  he-goat,  the  consort  of  which  is  also  a  common  two- 
horned  goat,  begets  a  kid,  from  the  skull  of  which  grow  four 
horns,  in  place  of  the  two  horns  previously  hereditary  in  this 
family  of  goats.  In  this  case  a  new  variety  of  goats  bear- 
ing four  horns  has  "  suddenly"  arisen,  and  under  favourable 


SUDDEN  VARIATION.  169 

conditions  this  young  he-goat  may  become  the  founder  of 
an  entirely  new  four-horned  race,  or  (by  correlative  adapta- 
tion and  constant  heredity)  of  a  new  fixed  species. 

But  if  we  now  search  for  the  physiological  functions  of 
evolution  which  have  "  suddenly  "  formed  this  new  race  or 
species,  we  find  that  a  change  in  the  hereditary  nutrition  at 
two  points  in  the  frontal  bone  and  in  the  skin  covering  the 
same  is  the  prime  cause.  Owing  to  the  excessive  local 
nutrition  of  the  osseous  tissue,  and  the  consequent  propor- 
tionate multiplication  of  cells,  a  bony  protuberance  gradually 
appears  at  each  of  these  points;  and  in  consequence  o* 
correlative  adaptation,  the  hairy  skin  covering  both  these 
protuberances,  changes  into  a  hard,  bare  horny  sheath, 
analogous  to  the  other  two  horns  which  have  long  been 
hereditary.  As  these  bony  protuberances  grow,  and  their 
horny  sheaths  become  correspondingly  larger,  a  new,  second 
pair  of  horns  appears  behind  the  old  ones.  All  these  func- 
tions of  evolution  which  "  suddenly  and  by  a  leap  "  produce 
this  four-horned  form  of  goat  are  in  reality  perfectly  "gradual 
and  continuous  "  changes  in  the  evolution  of  those  masses  of 
cells  of  which  we  have  spoken :  they  depend  on  a  change 
in  the  nutrition  of  the  tissue  at  these  two  points  in  the 
frontal  bone  and  skin.  In  this  instance,  therefore,  an  accu- 
rate examination  of  the  physiological  function  of  evolution 
affords  a  perfectly  natural  explanation  of  an  apparently 
miraculous  process.  This  is  equally  true  of  individual  and 
of  phyletic  evolution. 

This  is  also  the  explanation  of  a  process  of  evolution 
which  above  all  others  is  usually  put  under  mystical  veil 
as  though  it  were  a  supernatural  wonder;  this  is  the 
process  of  fertilization,  or  sexual  generation.  In  all  the 


I^O  THE   EVOLUTION   OF  MAN. 

higher  plants  and  animals  this  constitutes  the  first  act  in 
which  the  evolution  of  the  new  individual  begins.  But 
it  must  be  noted  here  that  this  important  process  is  by  no 
means  as  universally  distributed  throughout  the  animal  and 
vegetable  world  as  is  commonly  supposed.  On  the  contrary, 
there  are  very  many  low  organisms  which  always  multiply 
asexually,  e.g.  the  Amrebse,  Gregarinse,  Flagellata,  Forami- 
niferse,  Radiolaria.  Myxomycetse,  etc.  In  these  cases 
there  is  no  form  of  impregnation  :  the  multiplication  of 
individuals,  and  the  preservation  of  the  species  depend  here 
simply  on  asexual  generation,  under  the  forms  of  fission, 
propagation  by  buds  or  by  germ-cells.  On  the  other  hand, 
in  the  case  of  all  higher  plant  and  animal  organisms,  sexual 
propagation  is  the  general  law,  and  asexual  generation 
never  or  but  seldom  occurs.  Among  Vertebrates  in  par- 
ticular "virginal  generation"  (Parthenogenesis}  never 
occurs.  This  we  must  explicitly  affirm  in  the  face  of  the 
celebrated  dogma  of  the  "immaculate  conception."  "Im- 
maculate conception"  has  never  been  observed  either  in 
Man,  or  in  any  other  Vertebrate.4? 

Sexual  propagation  in  the  various  classes  of  animals 
and  plants  exhibits  an  especially  large  number  of  interest- 
ing correlations,  especially  those  relating  to  fertilization 
and  the  transmission  of  the  male  sperm  to  the  female  egg. 
These  correlations  are  of  the  utmost  significance  not  only  in 
regard  to  propagation,  but  also  in  the  production  of  organic 
bodily  forms,  and  especially  of  sexual  differences.  Very 
remarkable  instances  of  interaction  take  place  between 
plants  and  animals.  The  recent  admirable  researches  of 
Darwin  and  Hermann  Muller  on  the  fertilization  of  flowera 
by  insect  agency,  are  especially  interesting  from  this  point 


FERTILIZATION.  I/ 1 

of  view.49  As  a  result  of  this  interaction  we  find  a  sexual 
apparatus  of  very  complex  anatomy.  But  in  spite  of  the 
great  interest  of  these  phenomena,  we  cannot  discuss  them 
now,  as  they  are  only  of  subordinate  importance  in  study- 
ing the  essential  nature  of  the  process  of  fertilization.  On 
the  other  hand,  the  nature  of  this  process  itself — the  mean- 
ing of  sexual  generation,  must  be  closely  studied. 

In  every  process  of  fertilization,  as  has  already  been 
said,  two  different  kinds  of  cell,  male  and  female,  are  con- 
cerned. In  animals  generally  the  female  cell  is  called  the 
egg,  or  egg-cell  (ovulum),  and  the  male  is  called  the  sperm- 
cell,  or  seed-cell  (zoospermium,  spermatozoon).  The  female 
egg-cell,  the  form  and  structure  of  which  we  have  already 
considered,  is  in  all  animals  originally  of  the  same  simple 
structure.  At  first  it  is  simply  a  globular,  naked  cell, 
consisting  of  protoplasm  and  cell-nucleus  (Fig.  10,  p.  134). 
When  this  cell  lies  free,  and  is  capable  of  motion,  it 
performs  a  number  of  slow,  amoeboid  movements,  as  we 
have  seen  in  the  case  of  the  egg  of  the  Sponges  (Fig.  14, 
p.  144).  But  commonly  at  a  later  period  it  is  enclosed  in 
peculiar  envelopes  and  coatings  of  a  very  heterogeneous  and 
frequently  very  complex  structure.  On  the  whole,  the  egg- 
cell  is  one  of  the  largest  of  cells.  In  nearly  all  animals  it 
is  larger  than  any  of  the  other  cells. 

On  the  other  hand,  the  other  cell  which  plays  a  part  in 
impregnation,  the  male  sperm-cell,  is  one  of  the  smallest 
cells  of  the  animal  body.  As  a  rule,  fertilization  results 
from  a  mucous  fluid,  secreted  by  the  male,  coming  into 
contact  with  the  egg-cell,  either  within  or  without  the  body 
of  the  female.  This  fluid  is  called  the  sperm,  or  male  seed. 
The  sperm,  like  the  saliva  and  the  blood,  is  not  a  simple 
14 


172  THE    EVOLUTION    OF   MAN. 

clear  fluid,  but  a  dense  mass  of  exceedingly  numerous  cells, 
floating  about  in  a  comparatively  small  quantity  of  fluid. 
It  is  not  this  fluid,  but  the  cells  suspended  in  it,  which 
produce  fertilization.  In  most  animals,  these  sperm-cell? 
arc  possessed  of  two  special  properties.  In  the  first  place, 
they  are  extraordinarily  small,  usually  the  smallest  cells  in 
the  organism ;  and  secondly,  they  are  possessed  of  a  very 
peculiar  quick  motion  called  the  spermatozoid  movements. 
The  form  of  the  cells  is  in  correlation  with  this  movement. 
In  most  animals,  as  also  in  many  of  the  lower  plants  (but 
not  in  the  higher),  each  of  these  cells  consists  of  a  very 
small  naked  cellular  body,  enclosing  an  oblong  nucleus, 
and  of  a  long  vibrating  filament  attached  to  the  body  of 
the  cell  (Fig.  17).  It  was  a  very  long  time  before  it  was 
discovered  that  these  structures  are  simple  cells.  In  former 
times  they  were  universally  regarded  as  actual  animals, 
and  were  called  sperm-animals  (Spermatozoa).  It  is  only 
through  the  searching  investigations  of  the  past  few  years 
that  we  have  acquired  positive  evidence  of  the  fact  that 
each  of  these  so-called  spermatozoa  is  really  a  simple  cell. 
It  is,  therefore,  best  to  call  them  simply  seed-cells  or  sperm- 
cells.  In  Man  these  possess  the  same  form  as  in  many 
other  Vertebrates,  and  in  the  majority  of  Invertebrates. 
In  many  of  the  lower  animals,  however,  the  form  of  the 
seed-cells  is  very  different.  Thus,  for  example,  in  the  Cray- 
fish, they  are  fixed,  round  cells,  motionless,  and  furnished 
with  peculiar  stiff,  bristly  processes.  So,  too,  in  certain 
Worms,  e.g.  the  Thread-worms,  the  sperm-cells  possess  a 
very  anomalous  form.  Some  of  these  are  amoeboid,  re- 
sembling very  small  egg-cells.  Yet  even  in  most  of  the 
lower  animals,  e.g.  the  Sponges  and  the  Polyps,  they  possess 


SPERMATOZOA. 


173 


the    "pin-shaped   form"   which    occurs  in  Man  and  other 
Mammals  (Fig.  17). 


r    ~      i  i    IT-     J     IT        in  i    x     i    .ff 

FIG.  17. — Seed-cells  or  sperm-cells  from  the  semen  of  various  Mammals. 
1'ne  broad  side  of  the  flattened,  pear-shaped  nucleus  portion  of  the  sperm- 
cell  (the  so-called  "  head  of  the  sperm-animalcule")  is  represented  in  the 
drawings  marked  I;  the  narrow  side  in  those  marked  II  :  k,  kernel  of  the 
sperm-cell;  m,  central  portion  (protoplasm);  s,  active  tail-like  process 
(whip)  ;  M,  four  human  sperm-cells;  A,  two  sperm-cells  of  the. ape;  K,  of 
tlit-  rabbit;  H,  of  the  common  mouse;  C,  of  the  dog;  S,  of  the  pig. 

In  1G77,  when  the  Dutch  naturalist,  Leeuwenhoek,  first 
discovered  these  filamentous  and  very  active  tiny  bodies 
in  the  human  semen,  they  were  generally  supposed  to  be 
distinct,  independent  animalcules,  resembling  Infusoria,  and 
they  were  at  once  named  "  seminal  animalcules."  As  we 
have  already  observed,  they  played  an  important  part 
in  the  erroneous  theory  of  preformation  which  was  then 
prevalent,  according  to  which  the  whole  of  the  developed 
organism  with  all  its  parts  exists  preformed,  though  very 
small  and  as  yet  unexpanded,  in  each  seminal  animalcule. 
(See  p.  36.)  These  animalcules  had  only  to  penetrate 


!^4  THE  EVOLUTION   OF   MAN. 

into  the  fruitful  soil  of  the  female  egg-cell  in  order  that  the 
preformed  human  body  might  unfold  and  grow  in  all  its 
parts.  This  radically  erroneous  view  is  now  completely 
refuted,  and  the  most  accurate  researches  have  shown  that 
these  active  small  seminal  bodies  are  genuine  cells,  of  the 
form  called  flagellate.  In  the  earlier  expositions  of  the 
subject  a  head,  trunk,  and  tail  were  distinguished  in  each 
of  these  "  seminal  animalcules."  The  so-called  "  head  >; 
(Fig.  17  &)  is  only  the  longish  round  or  oval  cell-nucleus  . 
the  body,  the  central  portion  (m),  is  only  an  aggregation 
of  cell  material,  a  prolongation  of  which  forms  the  tail  (s). 
We  now  also  know  that  the  form  of  these  seminal  animal- 
cules is  not  even  peculiar  and  unrepresented  in  other  cells  , 
for  entirely  similar  vibratory  cells  occur  in  various  other 
parts  of  the  animal  body.  When  these  cells  are  possessed 
of  many  processes  they  are  called  ciliate  cells ;  but  if  they 
have  only  one  process,  they  are  said  to  be  flagellate.  The 
ciliated  sponge  particles  afford  instances  of  flagellate  cells 
resembling  those  of  the  sperm-cells. 

Thus  the  process  of  fertilization  in  sexual  generation 
depends  essentially  on  the  fact  that  two  dissimilar  cells 
meet  and  blend.  Tn  former  times  the  strangest  views  pre- 
vailed with  regard  to  this  act.  Men  have  always  been 
disposed  to  regard  it  as  thoroughly  mystical,  and  tho  most 
widely  different  hypotheses  have  been  framed  to  account 
for  it.  It  is  only  within  the  last  few  years  that  closer 
study  has  shown  that  the  whole  process  of  fertilization  is 
extremely  simple,  and  entirely  without  any  special  mystery. 
Essentially  it  consists  merely  in  the  fact  that  the  male 
sperm-cell  coalesces  with  the  female  egg-cell.  Owing  to  its 
sinuous  movements,  the  very  mobile  sperm-cell  finds  its  way 


THE   FERTILIZED   EGG-CELL.  175 

to  the  female  egg-cell,  penetrates  the  membrane  of  the  latter 
by  a  perforating  motion  and  coalesces  with  its  cell-material. 

FiG.lS.-Fertilizatiouoftheegg. 
cell  by  the  sperm-cells.  The  thread- 
shaped,  lively  sperm-cells  penetrate 
through  the  porous  canals  of  the  egg- 
membrane  into  the  granular  mass  of 
yelk,  with  which  they  amalgamate. 
The  kernel  (nucleus)  of  the  egg-cell 
has  disappeared. 

A  poet  might  find  in  this 
circumstance  a  capital  oppor- 
tunity for  painting  in  glowing 
colours  the  wonderful  mystery  of  the  process  of  fertiliza- 
tion;  he  might  describe  the  struggles  of  the  living  "seed- 
animalcules  "  eagerly  dancing  round  the  egg-cell  shut  up 
in  its  many  coverings,  disputing  the  passage  through  the 
minute  pore-canals  of  the  chorion,  and  then  "  of  purpose  " 
burying  themselves  in  the  protoplasm  of  the  yelk -mass, 
where,  in  a  spirit  of  self-sacrifice,  they  completely  efface 
themselves  in  the  better  "  ego."  Or  a  teleologist  might 
here  find  occasion  to  admire  the  peculiar  wisdom  of  the 
Creator,  who  made  many  fine  pore-canals  in  the  egg- 
membrane  in  order  that  the  seed-animalcules  might  pass 
through  them.  But  the  critical  naturalist  very  prosaically 
conceives  this  poetical  incident,  this  "  crown  of  love,"  as  the 
mere  coalescence  of  two  cells.  The  result  of  this  is  that,  in 
the  first  place,  the  egg-cell  is  rendered  capable  of  further 
evolution ;  and,  secondly,  that  the  hereditary  qualities  of 
both  parents  are  transmitted  to  the  child. 

The  fertilized  egg-cell  is,  therefore,  of  a  nature  entirely 
different  from  that  of  the  unfertilized  egg-cell.     For  since 


1/6  THE   EVOLUTION   OF  MAN. 

we  regard  the  sperm-cell  as  well  as  the  egg-cell  as  true  celb, 
and  since  fertilization  essentially  consists  in  the  amalgama- 
tion of  the  former  with  the  latter,  therefore  the  cell  which 
results  from  this  amalgamation  must  be  regarded  as  an  en- 
tirely new  independent  organism.  It  contains,  in  the  proto- 
plasm of  the  sperm-cell,  a  portion  of  the  paternal,  male  body, 
and  on  the  other  hand,  in  the  protoplasm  of  the  original 
egg-cell,  a  portion  of  the  maternal,  female  body.  This  is 
equally  shown  by  the  fact  that  the  child  inherits  many 
qualities  from  both  parents.  Heredity  from  the  father  is 
transmitted  through  the  sperm-cells,  Heredity  from  the 
mother  through  the  egg-cell  The  new  cell,  which  is  the 
rudiment  of  the  child,  the  newly  generated  organism, 
originates  in  an  actual  amalgamation  or  coalescence  of  the 
two  cells. 

In  order  to  gain  a  correct  and  clear  knowledge  ol 
fertilization,  I  think  it  is  absolutely  necessary  to  emphasize 
as  quite  fundamental  this  simple  but  most  important 
process,  which  as  yet  is  not  sufficiently  appreciated.  I  there- 
fore assign  a  peculiar  name  to  the  new  cell,  from  which 
the  child  really  proceeds,  and  which  is  usually  inaptly 
called  "the  fertilized  egg-cell"  or  "the  first  cleavage 
globule;"  I  shall  call  it  the  parent-cell  (cytula),  and  its 
kernel  (nucleus)  the  parent-kernel  (cytococcus).  The  name 
"  parent-cell "  seems  to  me  the  simplest  and  most  apt, 
because  all  the  other  cells  of  the  organism  descend  from  it, 
and  because  it  is  in  the  most  real  sense  both  the  male 
ancestor  and  the  female  ancestor  of  all  the  numerous 
generations  of  cells,  which  are  afterwards  employed  in  the 
formation  of  the  many-celled  organism.  The  very  complex 
molecular  movement  of  the  protoplasm  in  this  parent-cell, 


PARENT-CELL   AND    PARENT-KEEN  EL.  1/7 

summed  up  in  the  word  "life,"  is  naturally  entirely  dif- 
ferent from  that  of  the  two  distinct  ancestral  cells,  the 
amalgamation  of  which  gave  rise  to  the  parent-cell.  The 
life  of  the  parent-cell  (CYTULA)  is  the  product  or  resultant 
of  the-  paternal  activities,  transmitted  through  the  sp&rm- 
cell,  together  with  the  maternal  activities,  transmitted 
through  the  egg-cell. 

All  good  recent  observations  agree  in  showing  thai 
the  individual  evolution  of  man  and  of  other  animals 
begins  with  the  formation  of  such  a  parent-cell,  and  that 
in  the  course  of  further  evolution  this  then  separates 
by  self-division,  or  cleavage,  into  a  number  of  cells,  the 
so-called  cleavage-globules  or  cleavage- cells  (segmentella). 
But  the  most  active  strife  is  still  waged  over  the  question 
of  the  mode  in  which  the  parent-cell  (cytula]  originates, 
and  of  the  relative  parts  played  by  the  sperm-cell  and  the 
egg-cell  in  the  formation  of  the  parent-cell  and  in  the  act 
of  fertilization.  Formerly  it  was  usually  assumed — and 
many  well-known  naturalists  still  adhere  to  this— that  the 
original  kernel  (nucleus)  of  the  egg-cell  (p.  130,  Fig.  11), 
the  so-called  germ-vesicle,  is  retained  unaltered  during 
fertilization,  and  that  it  directly  transforms  itself  into  the 
parent-kernel,  "the  kernel  of  the  first  cleavage-globule." 
But  most  more  recent  observers  (with  whom  I  agree)  have 
become  convinced  that  the  germ-vesicle,  the  original  egg- 
kernel,  sooner  or  later  disappears,  and  that  the  parent- 
kernel  (cytococcus)  forms  itself  anew.  Here  again,  even 
the  question  as  to  the  time  and  mode  in  which  the  new 
kernel  of  the  parent-cell  forms  is  at  present  still  much 
debated.  Some  assume  that  the  germ-vesicle  disappears 
before  fertilization,  others  say  that  this  happens  after  ferti- 


1/8  THE   EVOLUTION    OF   MAN. 

lization.  One  party  affirms  that  it  is  expelled  from  the 
egg-cell,  the  other  that  it  dissolves  in  the  yelk.  Some  are 
of  opinion  that  it  disappears  entirely,  others,  that  it  only 
does  so  partially. 

We  cannot  here  enter  into  the  various  views  which  have 
recently  been  formed  as  to  this  remarkable  incident  in  fertili- 
zation, the  examination  of  which  presents  great  difficulties. 
Those  who  are  particularly  interested  in  it  may  be  referred 
to  valuable  works  on  this  subject  by  Auerbach,  Blitschli, 
Hertwig,  Strasburger,  and  others.50  Here  we  can  only 
briefly  indicate  the  view  which  at  present  appears  most 
probable.  Most  students  of  this  point  now  assume  as  a 
universal  incident  in  fertilization  that  the  germ- vesicle,  the 
original  kernel  of  the  egg-cell,  disappears  before  fertilization, 
being  either  expelled  from  the  egg  or  dissolved  in  the  yelk. 
Either  no  part  of  the  egg-cell,  or  only  the  germ-spot 
(nucleolus),  remains  as  a  defined  part  in  the  yelk.  Accord- 
ing to  Hertwig  and  others,  this  germ-spot  amalgamates  with 
the  sperm-kernel,  or  the  kernel  of  the  intruding  sperm-cell, 
and  this  amalgamation  gives  rise  to  the  kernel  of  the 
parent-cell.  On  the  contrary,  according  to  other  observers, 
the  parent-kernel  (cytococcus)  is  an  entirely  new  formation 
in  the  protoplasm  of  the  parent-cell  (cytula,  Fig.  21). 

At  present,  therefore,  the  majority  of  observers  assume 
that  between  the  original  nucleated  egg- cell  and  the 
known  nucleated  parent-cell  there  is  a  stage  in  which  there 
is  no  real  cell-kernel  or  nucleus,  and  in  which,  therefore,  the 
form- value  of  the  whole  organic  individual  is  no  longer  that 
of  a  true  nucleated  cell,  but  that  of  a  non-nucleated  cytod. 
i.e.  a  simple  protoplasmic  body  in  which  no  true  cell-kernel 
(nucleus)  is  to  be  found.  (Of.  p.  129.)  Even  if,  with  Hert- 


THE   MONERULA.  1 79 

xvig,  we  assume  that  the  germ-vesicle  does  not  completely 
disappear,  but  that  the  germ-spot  (nucleolus)  remains  and 
amalgamates  at  the  moment  of  fertilization  with  the 
nucleus  (or  nucleolus  ?)  of  the  sperm-cell,  we  may  say  that 
the  kernel  of  the  parent-cell  arises  anew  in  that  act,  and 
that,  therefore,  a  non-nucleated  germ-stage,  in  which  the 
form-value  of  the  germ  is  only  that  of  a  cytod,  precedes  the 
one-celled  germ-stage  (the  parent-cell).  For  reasons  which 
we  shall  presently  recognize,  we  shall  call  this  simplest 
(non-nucleated)  stage,  the  Monerula.51  (Fig.  19.) 


FIG.  19. — Monernla  of  a  Mammal  (Rabbit).  The  fertilized  egg-cell,  after 
the  disappearance  of  the  germ-vesicle,  is  a  simple  globe  of  protoplasm  (d). 
The  outer  membrane  is  formed  by  the  modified  zona  pellucida  (z) ,  together 
with  a  mucous  layer  (h)  secreted  on  the  outside  of  the  zona.  A  few  single 
sperm-cells  (s)  are  still  visible  in  the  membrane. 

We  regard  it  as  a  fact  of  the  greatest  interest  that  the 
human  child,  like  that  of  every  other  animal,  is,  in  this 
first  stage  of  its  individual  existence,  a  non-nucleated  ball 
of  protoplasm,  a  true  cytod,  a  homogeneous,  structureless 
body,  without  different  constituent  parts.  For  in  this 
"  Monerula-form  "  the  structure  of  the  animal,  and  thus  of 


180  THE   EVOLUTION   OF   MAN. 

the  human  organism,  is  of  the  simplest  conceivable  nature. 
The  simplest  actually  known  organisms,  and  at  the  same 
time  the  simplest  conceivable  organisms,  are  the  Monera, 
most  of  which  are  minute,  microscopic,  and  formless  bodies, 
consisting  of  a  homogeneous  substance,  of  an  albuminous  or 
mucous,  soft  mass,  and  which,  though  they  are  not  com- 
posed of  diverse  organs,  are  yet  endowed  with  all  the  vital 
qualities  of  an  organism.  They  move,  feed,  and  repro- 
duce themselves  by  division  (Fig.  20).  These  Monera 


FIG.  20.— A  Moneron  (Protamcebd)  in  the  act  of  reproduction.  A.  The 
whole  Moneron,  which,  like  the  Amoeba  (Fig.  13),  moves  by  means  of  change, 
able  processes.  B.  The  Moneron  is  pinched  in  at  a  central  point,  so  that  it 
is  divided  into  two  halves.  C.  The  two  halves  have  separated  and  each 
now  forms  an  independent  individual.  (Much  enlarged.) 

are  of  great  importance,  owing  to  the  fact  that  they 
afford  the  surest  starting-point  for  the  theory  of  the  origin 
of  life  on  our  earth.  We  shall  presently  have  further  oc- 
casion to  point  out  their  significance.  (Cf.  Chapter  XVI.) 
Here  we  need  only  give  due  weight  to  the  very  remarkable 
fact  that,  both  in  germ-history  and  in  tribal  history,  the 
animal  organism  begins  its  evolution  as  a  structureless 
mucous  ball.  The  human  organism,  like  that  of  the  higher 
animals,  exists  for  a  short  time  in  this  simplest  conceivable 
form,  and  its  individual  evolution  c  ommences  from  this 
simplest  form.  The  entire  human  child,  with  all  its  great 


CONSTITUENTS   OF   MONERULA. 


181 


future  possibilities,  is  in  this  stage  only  a  small,  simple  ball 
of  primitive  slime  (protoplasm,  Fig.  19).  The  membrane 
is  still  there,  but  seems  to  be  an  entirely  passive  part  of  the 
egg,  and  takes  no  real  share  in  the  active  processes  of  the 
evolution  of  this  egg.  We  may,  therefore,  for  a  time  pass 
over  this  membrane,  for  we  shall  afterwards  enter  into  the 
changes  which  it  undergoes  in  a  later  stage ;  as  regards  the 
actual  process  of  evolution,  it  is  entirely  without  significance. 
At  present  we  need  only  concern  ourselves  with  the  contents 


FIG.  21. — Parent-cell  or  cytula  of  a  Mammal  (Rabbit) :  Jc,  parent- 
kernel  ;  n,  nucleolus  of  the  latter ;  p,  protoplasm  of  the  parent-cell ;  z 
modified  zona  pellucida ;  s,  sperm-cells ;  h,  external  albuminous  membrane. 

of  the  globular  egg,  the  homogeneous  yelk,  which  when 
in  this  condition  we  call  the  Monerula,  in  allusion  to  the 
Monera-form. 

Although  morphologically  we  can  see  no  defined  con- 
stituent parts  in  the  Monerula,  yet  chemically  we  must 
regard  the  latter  as  the  complex  product  of  at  least  four 
different  constituents ;  these  are  :  (1)  the  protoplasm  of  the 
maternal  egg-cell ;  (2)  the  nrotoulasm  of  the  maternal 


1 82  THE   EVOLUTION   OF  MAN. 

sperm-cell ;  (3)  the  substance  of  the  maternal  germ-vesicle 
(kernel-substance  or  nuclein  of  the  egg-cell) ;  and  (4)  the 
substance  of  the  paternal  sperm-kernel  (kernel-substance  or 
nuclein  of  the  sperm-cell).  From  the  mixture  of  the  two 
former  substances  (1,  2)  the  protoplasm  of  the  parent-cell 
(Fig.  21,  p]  seems  to  originate  ;  from  the  mixture  of  the  two 
forms  (3,  4)  the  parent-kernel  (cytococcus)  seems  to  origin- 
ate (Fig.  21,  7c).52 

The  parent-cell  (cytula,  Fig.  21),  which  was  formerly 
regarded  as  merely  the  "  fertilized  egg-cell,"  differs  very 
essentially,  therefore,  from  the  original  egg-cell,  both  in 
point  of  form  (morphologically),  and  in  point  of  composition 
(chemically),  and  lastly,  also  in  point  of  vital  qualities 
(physiologically).  Its  origin  is  partly  paternal,  partly 
maternal;  we  need  not,  therefore,  be  surprised,  when  we 
see  that  the  child,  which  develops  from  this  parent-cell, 
inherits  individual  qualities  from  both  parents.53 

The  vital  activities  of  each  cell  form  a  sum  of  mechani- 
cal processes,  which  depend  radically  on  movements  of  the 
smallest  "life  particles,"  the  molecules  of  the  living  sub- 
stance. If  we  call  this  active  substance  the  Plasson,  and 
the  molecules  the  Plastidules,  we  may  say  that  the  indi- 
vidual physiological  character  of  each  cell  depends  on '  the 
molecular  movements  of  its  plastidules.  The  plastidule 
movements  of  the  cytula  are  therefore  the  resultant  of  the 
united  plastidule  'movements  of  the  female  egg-cell  xnd  of 
the  male  sperm-cell.  If  we  regard  the  two  latter  as  the 
sides  of  the  parallelogram  of  forces,  then  the  plastidule 
movement  of  the  cytula  is  the  diagonal.  In  my  work  on 
the  "Perigenesis  of  Plastidules"  (1876),  I  have  explained 
the  important  bearing  of  this  conception  in  explanation  of 
the  elementary  processes  of  evolution. 


TABLE   II. 


Review  of  the  Constituent  Parts  of  the  One-celled  Germ  .organism,  before 
and  after  fertilization. 

Of.  the  works  of  Ednard  Strasburger  ("  Ueber  Zellbildung,  Zelltheilung 
tnd  Befrnchtung,"  2nd  Edition  ;  Jena,  1876)  ;  of  Oscar  Hertwig  ("  Beitriige 
;ur  Kentniss  der  Bildung,  Befruchtung,  nnd  Theilang  des  Thierischen  Eies;" 
1875)  ;  of  Leopold  Auerbach  ("  Organologische  Studien  ;  "  1874)  ;  and  of  Otto 
Biitschli  ("  Studien  iiber  die  ersten  Entwickelungs-Vorgiinge  der  Eizelle," 
6tc.;  1876)." 


I.  The  Fertilizing  Male, 

II.  The  Fertilized  Female, 

III.  The  New  Cell,  the 

or     Paternal     Sexual 

or     Maternal     Sexual 

product  of  the  Concre- 

Cell. 

Cell. 

scence  of  I.  and  II. 

The  Sperm-cell. 

The  Egg-cell. 

The  Parent-cell. 

Spermule. 

Ovule. 

Cytula. 

Syn.       Thread  -  cell. 

Syn.  The  unfertilized 

Syn.      The   fertilized 

Seed-animalcule.    Sper- 

egg- 

egg.    The  first  cleavage- 

matozoa.     Zoosperm. 

Fig.  1,  p.  122. 

globule.       The      oldest 

Fig.  17,  p.  173. 

Fig.  10,  p.  134. 

cleavage-cell.      Segmen- 

tella  prinia. 

Fig.  21,  p.  181. 

Constituent  Parts. 

Constituent  Parts. 

Constituent  Parts. 

I.  A.  Protoplasm  of  the 

II.  A.  Protoplasm  of  the 

III.    A.    Protoplasm    of 

Sperm-cell. 

Egg-cell. 

Parent-cell:  Cleavage- 

yelk. 

(Spermop  la*ma.) 

(Ovoplasma.) 

(Gytuloplasma.) 

The    central    portion 

Yelk,  egg-yelk,  Lecy- 

Protoplasm     of     the 

and  the  tail  of  the  seed- 

thus,  vitellus. 

first     cleavage  -  globule 

thread,    together     with 

(the     product     of     the 

the  outer  sheath  of  the 

amalgamation  of  I.    A. 

"  head." 

and  II.  A. 

I.    B.   Kernel    (nucleus) 

II.  B.  Kernel  (nucleus) 

III.  B.  Kernel  (nucleus) 

of  the  Sperm-cell. 

of  the  Egg-cell. 

of  the  Parent-cell. 

(Spermococcus.) 

(Ovococcus.) 

(Cytulococcvs.) 

Sperm-kernel    (Hert- 

Germ -vesicle,  or  Pur- 

Cleavage-kernel  (Hert- 

wig).     "  Head    of    the 

kinje's  vesicle  (Vesicula 

wig).           Germ  -  kernel 

sperm-animal"       (with 

Germinativa),  contain- 

(Strasbnrger).    Kernel 

the    exception    of     the 

ing      the      germ-spot 

of    the    first    cleavage- 

tiiiu  outer  sheath). 

(Macula  Germinativa), 

globule  (product  of  the 

or  the  nucleolus,  which, 

amalgamation     of     the 

according  to  Hertwig, 

sperm  -kernel    and    the 

becomes  the  egg-kernel. 

egg-kernel  ?)." 

CHAPTER   VIII. 

EGG  CLEAVAGE  AND  THE   FORMATION    OF    THE   GERM- 
LAYERS. 


First  Processes  after  the  Fertilization  of  the  Egg-cell  is  complete.  — Original 
or  Palingenetic  Form  of  Egg-cleavage. — Significance  of  the  Cleavage- 
process. — Mulberry-germ,  or  Morula. — Germ-vesicle,  or  Blastnla.  Germ- 
membrane,  or  Blastoderm. — Inversion  (Invagination)  of  the  Germ-vesicle. 
— Formation  of  the  Gastrnla. — Primitive  Intestine  and  Primitive 
Month. — The  Two  Primary  Germ-layers;  Exoderm  and  Entoderm. — 
Kenogenetic  Form  of  Egg-cleavage. — Unequal  Cleavage  (aegmentatio 
inequalis)  and  Hood-gastrula  (Amphigastrula)  of  Amphibia  and 
Mammalia. — Total  and  Partial  Cleavage. — Holoblastic  and  Meroblastic 
Eggs. — Discoidal  Cleavage  (segmentatio  discoidalis)  and  Disc-gastrula 
(Disrogastrula)  of  Fishes,  Reptiles,  Birds. — Superficial  Cleavage  (seg- 
mentatio superficialis)  and  Vesicular  Gastrula  (Peri-Gastrula)  of  Ar- 
ticulates (Arthropoda). — Permanent  Two-layered  Body-form  of  Lower 
Animals.  —  The  Two-layered  Primaeval  Parent.form  ;  Gastrgea.  — 
Homology  of  the  Two  Primary  Germ-layers  in  all  Intestinal  Animals 
(Metaz''a). — Significance  of  the  Two  Primary  Germ-layers. — Origin 
and  Significance  of  the  Four  Secondary  Germ-layers. — The  Exoderm 
or  Skin-layer  gives  rise  to  the  Skin-sensory  Layer  and  the  Skin- 
fibrous  Layer.— The  Entoderm  or  Intestinal  Layer  gives  rise  to  the 
Intestinal-fibrous  Layer  and  the  Intestinal-glandular  Layer. 

"  The  distinguishing  of  the  strata,  or  layers,  in  the  embryonic  membrane 
was  a  turning-point  in  the  study  of  the  history  of  evolution,  and  placed 
later  researches  in  their  proper  light.  A  division  of  the  (disc-shaped) 
embryo  into  an  animal  and  a  plastic  part  first  takes  place.  When  this 
division  is  complete,  each  part  has  two  layers.  In.  the  lower  part  (the 
plastic  or  vegetative  layer)  are  a  serous  and  a  vascular  layer,  each  of  pecu- 


FORMATION   OF   GERM-LAYERS.  ^5 

liar  organization.  In  the  upper  part  also  (the  animal  or  serous  germ-layer) 
two  layers  are  clearly  distinguishable,  a  flesh-layer  and  a  skin-layer." — KARL 
ERNST  BAER  (1828). 

THE  first  processes  which  occur  in  the  evolution  of  the 
individual,  after  the  impregnation  of  the  egg-cell  is  com- 
plete, and  after  the  formation  of  the  parent-cell,  are  essen- 
tially similar  throughout  the  whole  animal  kingdom,  and 
always  begin  with  the  so-called  yelk -cleavage,  and  the 
formation  of  the  germ-layers.  Only  the  lowest  and  simplest 
animals,  the  Primaeval  Animals,  or  Protozoa,  are  peculiar  in 
this  respect.  These  latter  include  the  Monera,  Amoebae, 
Gregarinse,  Flagellata,  Rhizopoda,  Infusoria,  and  others. 
All  these  Primaeval  Animals  reproduce  themselves,  as  far  as 
we  yet  know,  only  asexually,  by  division,  the  formation  of 
buds,  spores,  germ-cells,  and  so  on.  On  the  other  hand,  they 
never  have  true  eggs,  i.e.  germ-cells,  to  the  evolution  ol 
which  fertilization  is  necessary.  Nor  do  they  ever  form 
true  germ-layers.  All  other  animals,  on  the  contrary,  all 
true  animals,  or  Metazoa  (as  we  may  call  them,  in  contra- 
distinction from  the  Protozoa)  have  true  eggs,  and,  from  their 
impregnated  eggs,  form  true  germ-layers.  This  is  as  true 
of  the  low  Plant-animals  and  Worms,  as  of  the  higher 
developed  Soft-bodied  animals  (Mollusca^)  Star-animals 
(Echinoderma),  Articulated  animals  (Arthropoda),  and  Ver- 
tebrates.55 

The  most  important  processes  of  germination  are  essen- 
tially similar  in  all  these  true  Animals  (the  Primaeval  animals 
being  excluded).  In  all,  the  parent-cell,  which  arose  from 
the  fertilized  egg-cell,  separates,  by  repeated  cleavage,  into 
a  large  number  of  simple  cells.  All  these  cells  are  direct 
followers  or  descendants  of  the  parent-cell,  and,  for  reasons 


\ 

1 86  THE   EVOLUTION   OF   MAN. 

which  will  be  explained  later,  are  called  Cleavage-cells  or 
Cleavage-globules  (segmentelld).  The  repeated  process  of 
division  of  the  parent-cell,  which  gives  rise  to  the  cleavage- 
cells,  has  long  been  known  as  egg-cleavage,  or,  inaccurately, 
as  cleavage  (segmentation).  At  an  earlier  or  later  stage,  the 
entire  mass  of  cleavage-cells  divides  into  two  essentially 
different  groups,  which  range  themselves  in  two  separated 
cell-strata ;  the  two  primary  germ-layers.  This  formation  of 
the  germ-layers  is  a  process  of  the  greatest  significance,  and 
the  real  beginning  of  the  formation  of  the  true  animal  body. 

It  is  only  quite  recently  that  the  fundamental  germinai 
processes  of  egg-cleavage  and  the  formation  of  the  germ- 
layers  have  been  thoroughly  understood,  and  their  real 
significance  rightly  estimated.  In  the  various  animal  groups 
these  processes  exhibit  various  striking  differences,  and  it 
was  no  easy  task  to  show  their  essential  similarity  or 
identity  throughout  the  whole  animal  kingdom  (always 
excepting,  of  course,  the  Primaeval  Animals,  or  Protozoa). 
It  was  only  after  I  had  established  the  Gastrsea  Theory,40 
in  1872,  and  afterwards,  in  1875,  had  traced  back  indi- 
vidual forms  of  egg-cleavage  and  of  the  formation  of  the 
gastrula  to  one  and  the  same  type-form,  that  this  important 
identity  could  be  regarded  as  really  proved.  This  furnished 
a  single  law  which  conditions  the  earliest  germinal  processes 
of  all  animals.56 

The  relation  of  Man  to  these  earliest  and  most  import- 
ant processes  is  entirely  similar  to  that  of  other  higher 
Mammals,  and  especially  to  -that  of  Apes.  As  the  human 
germ  or  embryo,  even  in  a  much  later  stage  of  its  formation, 
when  the  brain-bladders,  the  eyes,  the  organs  of  hearing, 
the  gill-arches,  etc.  are  also  present,  does  not  essentially 


EARLIEST   MAMMALIAN   GERM-PROCESSES.  l8/ 

differ  from  the  correspondingly  developed  embryos  of  other 
higher  Mammals  (Plate  VII.,  1st  row),  we  may  quite  safely 
assume  that  the  earliest  germinal-processes,  the  cleavage  of 
the  egg  and  the  formation  of  the  germ-layers,  also  corre- 
spond. As  yet,  however,  these  processes  have  not  been 
actually  observed  ;  for  there  has  never  been  an  opportunity 
of  dissecting  a  female  of  the  human  species  immediately 
after  fertilization  is  completed,  and  of  seeking  the  parent- 
cell,  or  the  cleavage-cells,  in  the  oviduct.  As,  however,  the 
youngest  human  embryo  (in  the  form  of  germ-vesicles), 
which  have  yet  been  really  observed,  as  well  as  the  subse- 
quently developed  germ-forms,  correspond  in  all  essential 
points  with  those  of  the  Rabbit,  the  Dog,  and  other  higher 
Mammals,  no  reasonable  man  can  doubt  that  egg-cleavage 
and  the  formation  of  the  germ-layers  proceeds,  in  the  one 
case  as  in  the  other,  in  the  way  represented  in  Plate  II. 
Fig.  12-17.57 

The  particular  form  which  egg-cleavage  and  the  forma- 
tion of  the  germ-layers  assume  in  the  case  of  Mammals,  is, 
however,  by  no  means  the  original,  simple,  and  palingenetic 
form  of  germination.  On  the  contrary,  it  has  been  very 
much  changed,  vitiated,  and  kenogenetically  modified  in 
consequence  of  numerous  embryonic  adaptations.  (Cf.  p.  12.) 
It  is,  therefore,  impossible  from  a  mere  study  of  it  to  learn 
its  nature.  On  the  contrary,  in  order  to  obtain  this  know- 
ledge, it  is  necessary  to  study  and  compare  the  various 
forms  of  egg-cleavage,  and  of  the  formation  of  the  germ- 
layers,  which  occur  in  the  animal  kingdom ;  and  it  is 
especially  necessary  to  search  for  the  original,  palingenetic 
form,  from  which  the  modified,  kenogenetic  form  of  germi- 
nation of  Mammals  gradually  arose  at  a  much  later  time. 
15 


[88  THE   EVOLUTION   OF   MAN. 

This  original,  palingenetic  form  of  egg-cleavage,  and 
of  the  formation  of  the  germ-layers  is  altogether  unrepre- 
sented in  the  present  day  in  the  Vertebrate  tribe,  to  which 
Man  belongs,  except  in  the  lowest  and  oldest  member  of 
this  tribe,  the  remarkable  Lancelot  or  Amphioxus  (Cf. 
Chapters  XIII.  and  XIV.,  and  Plates  X.  and  XI).  But  it 
is  still  found  in  exactly  this  form  in  many  low  inverte- 
brate animals — for  example,  in  the  remarkable  Sea-squirts 
(Ascidia),  in  the  Pond-snail  (Limnceus),  in  the  Arrow-worm 
(Sagitta) ;  also  in  many  Star-animals  (Echinoderma)  and 
Plant-animals, — for  example,  in  the  common  Star-fish  and 
Sea-urchin,  in  many  Medusae  and  Corals,  and  in  the 
simplest  Chalk  Sponges  (Olynthus).  As  an  example,  let  us 
examine  the  palingenetic  egg-cleavage  and  formation  of 
the  germ-layers  of  an  eight-rayed  single  Coral,  which  I 
found  in  the  Red  Sea,  and  described  in  my  Arabischen 
Korallen  under  the  name  of  Monoxenia  Darwinii.™ 

After  the  Monerula  (Fig.  22,  A)  has  changed  into  the 
parent-cell,  or  cytula  (B),  the  latter  divides  into  two  similar 
cells  ((7).  The  kernel  af  the  parent-cell  first  parts  into 
two  similar  halves ;  these  part  asunder,  shrink  from  each 
other,  and  then  act  as  centres  of  attraction  to  the  surround- 
ing protoplasm ;  after  this  the  protoplasm  becomes  con- 
tracted by  a  circular  groove  running  round  its  circumference, 
and  then  separates  into  two  similar  halves.  Each  of  the 
two  cleavage-cells,  which  are  thus  produced,  again  separates 
in  the  same  way  into  two  similar  cells,  the  plane  of  division 
between  these  two  latter  lying  at  right  angles  to  that 
between  the  two  former  (Fig.  22,  D).  The  four  similar 
cleavage-cells,  the  descendants  in  the  second  generation  of 
the  parent-cell,  lie  in  one  plane.  Each  of  these  now  again 


EGG-CLEAVAGE.  189 

divides  into  two  similar  halves,  the  division  of  the  cell- 
kernel  again  preceding  that  of  the  surrounding  proto- 
plasm. The  eight  cleavage-cells  thus  produced  bisect  in 
the  same  way  into  sixteen.  Thirty-two  cleavage  cells  are 
formed  from  these  by  further  division.  As  each  of  these 
again  bisects,  sixty-four  of  these  cells  are  produced ;  after- 
wards one  hundred  and  twenty-eight,  and  so  on.59  These 
repeated  and  similar  bisections  finally  result  in  the  produc- 
tion of  a  globular  mass  of  similar  cleavage-cells;  we  call 
this  mass  the  mulberry-germ  (morula}.  The  cells  lie  as 
close  together  as  the  drupes  of  a  mulberry  or  blackberry ; 
so  that  the  entire  surface  of  the  round  mass  appears  rugged 
(Fig.  22,  E).  (Cf.  Plate  II.  Fig.  3.60) 

After  this  egg-cleavage  is  completed,  the  solid  mulberry- 
germ  changes  into  a  hollow  globular  vesicle.  A  watery 
liquid  or  jelly  collects  in  the  centre  of  the  solid  ball ;  the 
cleavage-cells  part  asunder,  and  all  seek  the  surface  of  the 
ball.  Here  by  mutual  pressure  they  become  multilaterally 
flattened,  assume  the  form  of  truncated  pyramids,  and  range 
themselves  in  order,  side  by  side,  in  a  single  stratum 
(Fig.  22,  F,  Cr).  This  cell-stratum  is  called  the  germ-mem- 
brane (blastoderma) ;  the  cells  (all  of  one  kind),  a  simple 
stratum  of  which  forms  the  germ-membrane,  are  called  the 
germ-membrane-cells  (cellulce  blastodermicce) ;  and  the  entire 
hollow  ball,  the  walls  of  which  are  composed  of  these  cells, 
is  called  the  germ-membrane-vesicle,  or,  briefly,  the  germ- 
vesicle,  or  vesicular-germ  (blastula ;  formerly  called  the 
vesicula  Uastodermica).61  The  inner  cavity  of  the  ball, 
which  is  filled  with  clear  liquid  or  jelly,  is  called  the 
cleavage-cavity  (cavum  segmentarium),  or  the  germ-cavity 
(blastocodoma}. 


INVERSION.  IQl 

FtG.  22. — Germination  of  a  Coral  (Monoxenia  Darwinii)  :  A,  Monerulaj 
B,  Parent-cell  (Cytula) ;  C,  two  cleavage-cells;  D,  fonr  cleavage-cells; 
E,  Mulberry-germ  (Morula)  ;  F,  the  Germinal  vesicle  (Elastula)  ;  G,  Ger- 
minal vesicle  in  section ;  II,  Germinal  vesicle  (inverted)  in  section ;  I, 
Gastrula  in  longitudinal  section;  K,  Gustrula,  or  Germ-cup,  seen  from 
outside. 

In  this  Coral,  as  in  many  other  low  animals,  the  young 
animal-germ  begins  to  move  even  in  this  stage,  and 
swims  about  independently  in  the  water.  A  long,  thin, 
thread-like  process,  a  whip  or  thong,  grows  out  from  each 
of  the  cells  of  the  germ-membrane ;  and  these  inde- 
pendently exert  slow  vibrations,  which  afterwards  be- 
come quicker  (Fig.  22,  F).  Each  cell  of  the  germ-membrane 
is  thus  transformed  into  a  vibrating  whip-cell.  The  whole 
globular  germ-vesicle  revolves  or  turns,  and  is  driven  about 
in  the  water  by  the  united  force  of  all  these  vibrating  whip- 
processes.  In  many  other  animals,  especially  in  those  in 
which  the  germ  is  developed  within  closed  egg-membranes, 
the  vibrating  whip -threads  on  the  <jells  of  the  germ -mem- 
brane are  not  developed  till  a  later  period,  or,  even,  are  not 
formed  at  all.  The  germ-vesicle  is  capable  of  growing  and 
extending,  for  the  cells  of  the  germ-membrane  increase  by 
repeated  division,  which  occurs  within  the  surface  of  the 
ball,  and  more  liquid  is  secreted  in  the  centre  cavity. 

A  most  important  and  remarkable  process  now  occurs ; 
this  is  the  inversion  of  the  germ-vesicle  (invaginatio  blas- 
tulce,  Fig.  22,  E}.  The  ball,  the  wall  of  which  is  cellular, 
consisting  of  a  single  layer,  changes  into  a  cup  with  a  two- 
layered  cellular  wall.  (Of.  Fig.  22,  G,  H,  I.)  The  outer  sur- 
face of  the  ball  becomes  flattened  at  a  particular  point ;  and 
this  flattening  deepens  into  a  groove.  The  groove  becomes 
deeper  and  deeper,  growing  at  the  expense  of  the  central 


IQ2  THE   EVOLUTION   OF   MAN. 

germ-cavity,  or  cleavage-cavity.  The  latter  decreases  in 
proportion  as  the  former  extends.  At  last  the  central  germ- 
cavity  entirely  disappears,  while  the  inner,  inverted  portion 
of  the  germ-membrane,  the  wall  of  the  groove,  attaches  its 
inner  surface  to  the  inner  surface  of  the  outer,  uninverted 
portion  of  the  germ-membrane.  At  the  same  time,  the  cells 
of  the  two  parts  assume  a  different  form  and  size  ;  the  inner 
<;ells  become  rounder  ;  the  outer  become  longer  (Fig.  22,  /). 
The  germ  thus  acquires  the  form  of  a  cup  or  goblet- 
shaped  body,  the  wall  of  which  consists  of  two  different 
cell-layers,  while  the  cavity  in  its  centre  grows  outward  at 
one  end,  at  the  place  where  the  inversion  originated.  This 
highly  important  and  interesting  germ-form  is  called  the 
germ-cup  or  the  intestinal  larva  (Gastrula,  Fig.  22,  /,  in 
longitudinal  section ;  K,  surface  view).62 

The  Gastrula  seems  to  me  the  most  important  and 
significant  germ-form  of  the  animal  kingdom.  For  in  all 
true  animals,  the  Protozoa  excepted,  the  egg-cleavage 
results  either  in  a  genuine,  original,  palingenetic  gastrula 
(Fig.  22,  /,  K),  or  in  an  equivalent  kenogenetic  germ- 
form,  which  has  arisen  secondarily  out  of  the  earlier  form, 
and  which  may  be  referred  directly  back  to  that  form. 
V.  is  certainly  a  most  highly  interesting  and  significant  fact, 
that  animals  of  the  most  diverse  tribes,  Vertebrates,  Soft- 
bodied  Animals  (Mollusca),  Articulated  animals  (Arthro- 
poda),  Star-animals  (Echinoderma),  Worms,  and  Plant- 
animals  (Zoophyta)  develop  from  one  common  germ-form. 
In  most  striking  illustration  of  this,  I  place  side  by  side 
several  genuine  Gastrula  forms,  taken  from  tribes  of  animals 
(Fig.  23-28,  with  the  description). 

This  extraordinary  importance  of  the  Gastrula  makes 


THE   GASTRULA 


193 


it  necessary  that  we  should   most   carefully  examine  the 

structure  of  its  body.     Ordinarily   it   is   invisible   to   the 

FIG.  24.  FIG.  25.  FIG.  26.  FIG.  27. 


Flu.  23. 


FIG.  28. 


FIG.  23. — (A)  Gastrnla  of  a  Zoophyte  (Gastrophysema).     (Haeckel.) 

FIG.  24.— (B)  Gastrula  of  a  Worm  (Sagitta).     (After  Kowalevsky.) 

FIG.  25.— (C)  Gastrula  of  an  Echinoderm  (Starfish,  Uraster).  (After 
Alexander  Agassiz.) 

FIG.  26. —  (D)  Gastrula  of  an  Arthropod  (Nauplius). 

FIG.  27.— (E)  Gastrula  of  a  Mollusc  (Pond-snail,  Limnceus).  (After  Karl 
fiabl.) 

FIG.  28.- (F)  Gastrula  of  a  Vertebrate  (Lancelot,  Amphioxus).  (After 
Kowalevsky.) 

In  all,  d  indicates  the  primitive  intestinal  cavity;  o,  the  primitive  mouth; 
s,  the  cleavage-cavity  ;  i,  the  entoderrn,  or  intestinal  layer;  e,  the  exoderm, 
or  skin-layer. 


194  THE   EVOLUTION   OF  MAN. 

naked  eye,  or,  at  most,  under  favourable  circumstances,  it 
is  seen  as  a  tiny  speck,  usually  ^ — ^,  or  at  most  ^ — \ 
millimetre  in  diameter ;  it  is  hardly  ever  more.  In  form 
the  body  of  the  Gastrula  is  usually  cup-like ;  sometimes  it 
is  rather  egg-shaped,  sometimes  rather  ellipsoid  or  fusiform  ; 
in  other  cases  it  is  more  hemispherical,  or  almost  spherical ; 
and  again  in  others,  longer  or  almost  cylindrical.  The 
geometric  outline  of  the  body  is  highly  characteristic;  it 
is  marked  by  a  single  axis  with  two  differing  poles.  This 
axis  is  the  main,  or  longitudinal  axis  of  the  future  animal 
body ;  one  pole  is  the  mouth,  or  oral  pole ;  the  opposite  is 
the  aboral  pole.  This  outline  with  one  axis  distinguishes 
the  Gastrula  very  essentially  from  the  globular  Blastula 
and  Morula,  in  which  all  the  axes  of  the  body  are  similar.63 
I  shall  call  the  central  cavity  of  the  Gastrula-body  the 
primitive  intestine  (protogaster],  and  its  opening  the  pri- 
mitive mouth  (protosto-ma).  For  this  cavity  is  the  original 
nutritive,  or  intestinal  cavity  of  the  body,  and  this  opening 
originally  served  to  admit  food  into  the  body.  It  is  true 
that  at  a  later  period  the  primitive  intestine  and  the 
primitive  mouth  appear  very  different  in  the  different 
tribes  of  animals.  This  is  especially  true  of  Vertebrates, 
in  which  only  the  middle  portion  of  the  later-formed  in- 
testinal canal  proceeds  from  the  primitive  intestine;  and 
,in  which  the  later  mouth-opening  is  a  formation  entirely 
independent  of  the  primitive  mouth,  which  closes.  It  is, 
therefore,  necessary  to  distinguish  clearly  between  the 
primitive  mouth  and  intestine  of  the  Gastrula  on  the  one 
hand,  and  the  later-formed  intestine  and  mouth  of  the 
leveloped  Vertebrate  on  the  other.64 

The  two  cellular  layers   which  surround  the  cavity  of 


STRUCTURE  OF  THE  GASTRULA. 


195 


the  primitive  intestine,  and  alone  constitute  the  wall  of 
the  latter,  are  of  very  great  significance.  For  these  two 
which  alone  constitute  the  whole  body,  are,  in  fact,  the 
two  primary  germ-layers,  or  primitive  germ -layers  (blas- 
tophylla).  Their  fundamental  significance  has  already  been 
pointed  out  in  the  historical  introduction  (Chapter  III.). 
The  outer  cell-layer  is  the  skin-layer,  or  exoderm  (Fig.  29,  e) ; 
the  inner  cell-layer  is  the  intestinal  layer,  or  entoderm 
(Fig.  29,  e}.  The  whole  body  of  all  true  animals  proceeds 
solely  from  these  two  primary  germ-layers.  The  skin- 
layer  furnishes  the  outer  body- wall ;  the  intestinal  layer 
forms  the  inner  wall  of  the  intestine,  and  directly  surrounds 
the  intestinal  cavity.  At  a  later  period  a  cavity  forms 


FIG.  29. — The  Gastrula  of  a  Chalk  Sponge  (Olynfhus)  :  A,  external  view. 
B,  in  longitudinal  section  through  the  axis  ;  g,  primitive  intestine  (primitive 
intestinal  cavity)  ;  o,  primitive  mouth  (primitive  mouth-opening)  ;  i,  the 
inner  cell-layer  of  the  body-wall  (the  inner  germ-layer,  entoderm  or  intes- 
tinal layer)  ;  e,  the  outpi-  cell -layer  (the  outer  germ -layer,  exoderm  or  skin- 
layer). 


196  THE   EVOLUTION   OF  MAN. 

between  the  two  germ-layers ;  this  cavity,  filled  with  blood 
or  lymph,  is  the  body-cavity  (cceZoma).66 

The  two  primary  germ -layers,  the  outer  or  serous,  and 
the  inner  or  mucous  layer,  were  first  clearly  distinguished, 
in  1817,  by  Pander,  in  the  incubated  Chick  (p.  51).     But 
their  full  significance  was  first  thoroughly  recognized  by 
Baer,  who,  in  his  "  History  of  Evolution  "  (1828),  gave  the 
name  of  animal  layer  to  the  outer  layer,  that  of  vegetative 
layer  to  the  inner.     These  names  are  very  apt,  because  it  is 
the  outer  layer  which  especially  (if  not  exclusively)  gives  rise 
to  the  animal  organs  of  sensation  and  movement,  the  skin, 
the  nerves,  and  the  muscles ;  while,  on  the  other  hand,  it  is 
especially  from  the  inner  layer  that  the  vegetative  organs 
of  nourishment  and  reproduction,  the  intestine  and  blood- 
vessel  system   in   particular,  arise.     Twenty   years   after- 
wards   (in  1849)  Huxley  pointed   out   that  in  many  low 
Plant-animals  (Zoophyta),  such  as  the  Medusre,  the  whole 
body   permanently   consists    only   of   these    two    primary 
germ-layers.     The  outer  of  these  he  called  the  ectoderm,  or 
exoderm ;  the  inner  he  named  the  endoderm,  or  entoderm. 
Recently  Kowalevsky  and  Ray  Lankester  especially  have 
tried  to   show    that   other    Invertebrate    animals  of    the 
most  diverse  classes,  in  Worms,  Soft-bodied  Animals  (Mol- 
lusca),  Star-animals  (Echinoderma),and  Articulated  animals 
(Arthropoda),  form   from   the   same   two    primary    germ- 
layers.     Lastly,  I  have  myself  shown  that  this  is  the  case 
also  in  the  lowest  Plant-animals,  in  Sponges ;  and  at  the 
same  time  I  tried  to  prove  in  my  Gastrsea  Theory  that  these 
two  primary  germ-layers  must  be  considered  as  of  the  same 
significance,  or  as  homologous,  in  all  cases,  from  Sponges 
and  Corals  to  Insects  and  Vertebrates,  including  Man. 


BLASTODERMIC   CELLS.  1 97 

Ordinarily  the  cells  of  the  Gastrula-germ,  which  com- 
pose the  two  primary  germ-layers,  already  present  recog- 
nizable differences.  In  most  cases,  if  not  in  all,  the  cells 
of  the  skin-layer,  or  exoderm  (Fig.  29,  e),  are  smaller,  more 
numerous,  and  brighter  coloured;  on  the  other  hand,  the 
cells  of  the  intestinal  layer,  or  entoderm  (Fig.  29,  i),  are 
larger,  less  numerous,  and  darker.  The  protoplasm  of  the 
exoderm  cells  is  clearer  and  firmer  than  the  darker  and 
softer  cell-substance  of  the  entoderm  cells;  the  latter  are 
generally  much  richer  than  the  former  in  fatty  particles. 
The  cells  of  the  intestinal  layer  usually  also  have  a  much 
greater  affinity  for  colouring  matter,  and  take  up  carmine, 
aniline,  and  so  on,  from  solution  much  more  quickly  and 
vigorously  than  do  the  cells  of  the  skin-layer. 

These  physical,  chemical,  and  morphological  differences 
in  the  two  germ-layers  correspond  to  their  physiological  dif- 
ferences, and  are  of  great  interest,  because  in  them  we  see  the 
first  and  earliest  process  of  division  or  differentiation  of  the 
animal  body.  The  germ-membrane  (blastoderma),  which 
forms  the  wall  of  the  globular  germ-vesicle,  or  Blastula 
(Fig.  22,  F,  G),  consisted  solely  of  a  single  layer  of  similar 
cells.  These  cells  of  the  germ-membrane,  or  blastoderm,  are 
usually  formed  in  a  very  regular  and  even  way,  and  are  of 
entirely  similar  size,  form,  and  qualities.  Generally  they 
are  flattened  by  pressing  against  each  other,  and  are  often 
uniformly  six-sided.  This  uniformity  of  the  cells  disap- 
pears, at  an  earlier  or  later  period,  during  the  inversion 
(invaginatio)  of  the  germ-vesicle.  The  cells,  composing 
the  inverted,  inner  part  of  the  germ-vesicle  (which  after- 
wards form  the  entoderm)  usually  assume,  even  during  the 
process  of  inversion  (Fig.  22,  H],  a  nature  differing  from 


IQ8  THE   EVOLUTION   OF   MAX. 

that  of  the  cells  which  constitute  the  outer,  uninverted  part 
(the  future  exoderm).  When  the  process  is  completed,  the 
histological  differences  in  the  cells  of  the  two  primary 
germ-layers  are  usually  very  strongly  marked  (Fig.  30). 
The  small,  bright-coloured  cells  of  the  exoderm  (e)  are 
clearly  distinguishable  from  the  larger,  darker  cells  of  the 
entoderm  (i}. 

FIG.  30. — Cells  from  the  two  primary  germ, 
layers  of  a  Mammal  (from  the  two  strata  of 
the  germ-membrane)  :  i,  the  larger,  darker 
cells  of  the  inner  stratum,  the  vegetative 
germ-layer, or  entoderm;  e,the  small,  brighter- 
coloured  cells  of  the  outer  stratum,  the  animal 
germ-layer,  or  exoderm. 


At  present  we  have  only  con- 
sidered that  form  of  egg-cleavage,  of 
germ -layer  and  gastrulation,  which 
on  many  and  important  grounds  we  are  justified  in  regard- 
ing as  the  original,  primary,  and  palingenetic  form.  We  call 
this  the  primordial,  or  original,  form  of  egg-cleavage  ;  and 
the  Gastrula,  resulting  from  this,  we  call  the  Bell-gastrula 
(Archigastrula).  In  a  form  exactly  similar  to  that  of  our 
Coral  (Monoxenia,  Fig.  22),  we  meet  with  this  Bell-gastrula 
in  the  lowest  Plant-animals,  in  the  Gastrophysema  (Fig.  23), 
also  in  the  simplest  Chalk  Sponges  (Olynthus,  Fig.  29), 
in  many  Medusae  and  Hydra-polyps ;  in  low  Worms  of  dif- 
ferent classes  (Sagitta,  Fig.  24 ;  Ascidia,  Plate  X.  Fig.  1-4) ; 
again,  in  many  Star-animals  (Echinoderma,  Fig.  25) ;  in 
low  Articulated-animals  (Arthropoda,  Fig.  26),  and  Soft- 
bodied  Animals  (Mollusca,  Fig.  27) ;  lastly,  in  the  lowest 
Vertebrate  (Amphiotnis,  Fig,  28;  Plate  X.  Fig.  7-10). 


BELL-GASTRULA.  199 

Although  the  animals  which  we  have  named  belong  to 
the  most  diverse  classes,  they  all  have  this  in  common 
-with  each  other  and  with  many  other  animals,  that,  owing 
to  constant  heredity,  they  have  retained  the  palingenetic 
form  of  egg-cleavage  and  Gastrula-formation,  which  they 
received  from  their  oldest  common  ancestors,  up  to  the  pre- 
sent day.  This  is,  however,  not  true  of  the  large  majority 
of  animals.  On  the  contrary,  in  them  the  original  process 
of  germination  has,  in  the  course  of  many  million  years, 
gradually  changed  in  a  greater  or  less  degree,  and  has 
become  vitiated  owing  to  adaptation  to  new  conditions  of 
evolution.  Both  the  egg-cleavage,  or  segmentation,  and 
the  formation  of  the  Gastrula,  or  gastrulation,  which 
succeeds  the  segmentation,  have  in  consequence  of  this 
acquired  an  aspect  which  is  in  many  ways  different.  In 
the  course  of  time  the  differences  have  even  become  so 
marked  that  the  cleavage  process  of  most  animals  was 
wrongly  interpreted,  and  the  Gastrula  of  these  animals  was 
altogether  unknown.  It  is  only  owing  to  the  extensive 
comparative  researches  which  I  instituted  in  late  years 
among  animals  of  the  most  diverse  classes,  that  I  have  been 
enabled  to  indicate  the  one  common  process  which  under- 
lies those  processes  of  germination,  apparently  so  different, 
and  have  traced  back  all  the  diverse  forms  of  germination 
to  the  one  original  form,  the  form  which  has  already  been 
described.  To  distinguish  them  from  this  primary  palin- 
genetic  form  of  germination,  I  shall  call  all  the  secondary 
forms,  varying  from  the  primary,  vitiated,  or  kenogenetic 
processes.  The  more  or  less  varying  Gastrula-form,  which 
results  from  this  kenogenetic  egg-cleavage,  may  be  called, 
generally,  the  secondary,  modified  Gastrula,  or  Metagastnda.. 


20O  THE   EVOLUTION   OF   MAN. 

Among  the  many  and  various  kenogenetic  or  vitiated 
forms  of  egg-cleavage  and  gas tr illation,  I  again  distinguish 
three  different  chief  forms :  1.  Unequal  cleavage  (segmen- 
tatio  incequalis,  Plate  II.  Fig.  7-17) ;  2.  Discoidal  cleavage 
(segmentatio  discoidalis,  Plate  III.  Fig.  18-24);  and  3. 
Surface  cleavage  (segmentatio  superficialis,  Plate  III.  Fig. 
25-30).  Unequal  cleavage  results  in  a  Hood-gastrula 
(Amphigastrula,  ^ate  II.  Fig.  11  and  17);  discoidal  cleavage 
results  in  a  Disc-gastrula  (Discogastrula,  Plate  III.  Fig.  24) ; 
surface  cleavage  results  in  a  Bladder-gastrula  (Perigastrula, 
Plate  III.  Fig.  29).  The  last  form  does  not  occur  among 
Vertebrates,  with  which  we  are  now  specially  concerned; 
it  is,  on  the  contrary,  the  usual  form  among  Articulated 
Animals  (Spiders,  Crabs,  Insects,  etc.).  In  Mammals  and 
Amphibia  the  cleavage  is  unequal,  and  the  Gastrula  is  a 
Hood-gastrula ;  this  is  equally  true  of  the  Ganoid  fish  and 
the  Round-mouths  (Lampreys  and  Hagfishes).  On  the  other 
hand,  in  most  Fishes,  and  in  all  Reptiles  and  Birds,  we  find 
the  discoid  form  of  cleavage,  and  a  Disc-gastrula.  (Cf. 
Table  III.) 

As  Man  is  a  true  Mammal,  and  as  human  germination 
is  entirely  similar  to  that  of  other  Mammals,  the  cleavage 
in  his  case  also  is  unequal,  and  results  in  the  formation  of  a 
Hood-gastrula  (Amphigastrala,  Plate  II.  Fig.  12-17).  But 
it  is  peculiarly  difficult  to  investigate  the  first  incidents  in 
the  egg-cleavage  and  gastrulation  of  Mammals.  It  is  true 
that  more  than  thirty  years  ago  the  anatomist  Bischoff,  of 
Munich,  laid  a  foundation  for  this  work  in  two  books,  which 
he  published,  on  the  germ-history  of  the  Rabbit  (1842),  and 
on  that  of  the  Dog  (1845);  and  that  these  were  afterwards 
followed  by  two  equally  careful  studies  of  thb  germination 


LATER  FORMS  OF  GASTRULA.  2OI 

of  the  Guinea-pig  (1852),  and  of  the  Roe-deer  (1854).  But  it 
was  only  quite  recently  that  Eduard  van  Beneden,  an  emi- 
nent Belgian  zoologist,  was  able,  owing  to  the  elaborated 
methods  of  preparation  of  the  present  day,  to  throw  full 
light  on  the  obscurity  which  surrounded  the  germination  of 
Vertebrates,  and  to  give  a  right  explanation  of  its  details. 
It  still,  however,  remains  so  difficult  to  understand  these 
details,  that  it  is  desirable  to  glance  first  at  the  germination 
of  Amphibia.  In  common  with  Mammals,  these  animals 
exhibit  unequal  cleavage,  and  form  a  Hood-gastrula.  But 
the  details  of  germination  are  simpler  and  more  evident  in 
Amphibia  than  in  Mammals,  and  they  are  more  nearly  akin 
to  the  original,  palingenetic  form  of  germination. 

The  eggs  of  the  common,  tailless  Amphibia,  of  the  Frog 
and  the  Toad,  afford  the  best  and  most  convenient  objects 
for  this  examination.  Masses  of  them  are  easily  obtainable 
in  the  spring  from  all  ponds  and  pools ;  and  a  careful 
examination  of  the  eggs  with  a  magnifying  glass  is  suffi- 
cient to  show  at  least  the  external  features  of  the  egg- 
cleavage.  In  order,  however,  to  obtain  a  correct  idea  of  the 
intricate  details  of  the  whole  process,  and  to  understand  the 
formation  of  the  germ-layers  and  of  the  gastrula,  the  egg  of 
the  Frog  must  be  carefully  hardened, ,  and,  the  thinnest 
possible  sections  having  been  cut  with  a  razor  from  the 
hardened  egg,  these  must  be  most  minutely  examined  under 
a  powerful  microscope.66 

The  eggs  of  the  Frog  and  of  the  Toad  are  globular  in 
form,  and  have  a  diameter  of  about  two  millimetres  ;  they 
are  laid  in  great  numbers  in  masses  of  jelly,  which,  in 
the  case  of  the  Frog,  form  thick  lumps,  while  those  of  the 
Toad  form  long  strings.  When  the  opaque,  brown,  grey, 


202  THE   EVOLUTION   OF   MAN. 

or  black-coloured  egg  is  minutely  examined,  the  upper 
half  appears  darker  than  the  lower.  In  some  kinds,  the 
centre  of  the  upper  half  is  blacker,  while  the  corresponding 
centre  of  the  lower  half  is  of  a  whiter  colour.67  This  marks 
a  distinct  axis  of  the  egg  with  two  different  poles.  In  order 
to  give  a  clear  conception  of  the  cleavage  of  this  egg,  it  is 
best  to  compare  it  to  a  globe,  on  the  surface  of  which 
different  meridian  and  parallel  circles  are  marked.  For 
the  superficial  boundary  lines  between  the  different  cells, 
which  result  from  repeated  division  of  the  egg-cell,  have 
the  appearance  of  deep  furrows  on  the  surface,  for  which 
reason  the  whole  process  has  received  the  name  of  "the 
furrowing"  (i.e.  cleavage).59  But  this  so-called  cleavage, 
which  was  formerly  regarded  with  astonishment  as  a  very 
wonderful  process,  is,  in  reality,  only  an  ordinary  and  often- 
repeated  division  of  the  cells.  Therefore  the  "  cleavage- 
globules,"  which  result  from  it,  are  really  true  cells. 

Unequal  cleavage,  as  we  see  it  in  the  amphibian  egg,  is 
especially  marked  by  the  fact  that  it  begins  at  the 
upper,  darker  pole — the  north  pole  of  the  globe,  according  to 
our  simile — and  proceeds  slowly  downwards  towards  the 
lower,  lighter  pole,  the  south  pole.  During  the  egg-cleav- 
age the  upper,  darker  hemisphere  is  in  advance,  and  its 
cells  divide  more  vigorously  and  quickly ;  the  cells  of  the 
lower  hemisphere,  therefore,  appear  larger  and  less  numer- 
ous.67 The  cleavage  of  the  parent-cell  (Fig.  31,  A )  begins 
with  the  formation  of  an  entire  meridian-furrow,  which 
starts  at  the  north  pole  and  ends  at  the  south  pole  (B). 
An  hour  later,  a  second  meridian-furrow  arises  in  the  same 
way,  and  cuts  the  first  at  right  angles  (Fig.  31,  C).  The 
sphere  of  the  egg  is  thus  divided  into  four  similar  segments. 


FORMS   OF   CLEAVAGE. 


203 


Each  of  these  four  first  cleavage-cells  consists  of  an  upper, 
darker,  and  of  a  lower,  brighter  half.  A  few  hours  after- 
wards a  third  furrow  appears,  perpendicularly  to  the  two 


FIG.  31.— The  cleavage  of  a  Frog's  egg  (10  times  enlarged)  :  A,  the 
parent-cell;  B,  the  two  first  cleavage-cells;  C,  4  cells;  D,  8  cells  (4 
animal  and  4  vegetative) ;  E,  12  cells  (8  animal  and  4  vegetative)  ;  F, 
16  cells  (8  animal  and  8  vegetative) ;  G,  24  cells  (16  animal  and  8  vege. 
tative) ;  H,  32  cells ;  I,  48  cells ;  K,  64  cells ;  L,  96  cleavage-cells ;  M, 
160  cleavage-cells  (128  animal  and  32  vegetative). 

former  (Fig.  31,  D}.      This   ring-furrow   is   generally,  but 
wrongly,  called  the  "  equatorial  furrow ; "  it  lies  north  from 

the  equator,  and  should,  therefore,  rather  be  compared  to  the 
16 


204  THE   EVOLUTION   OF   MAN. 

northern  tropical  line.  The  spherical  egg  now  consists  of 
8  cells,  4  smaller,  upper,  or  northern,  and  4  larger,  lower, 
or  southern.  A  meridian-furrow,  starting  from  the  northern 
pole,  now  appears  in  each  of  the  first  four  cells,  each  of 
which  falls  into  two  similar  halves,  so  that  8  upper  cells 
lie  on  4  lower  cells  (Fig.  31,  E).  It  is  only  later  that  the 
four  new  meridian  cells  place  themselves  slowly  on  the 
lower  cells,  so  that  the  number  mounts  from  12  to  16  (F). 
Parallel  to  the  first,  horizontal  ring-furrow,  a  new  ring- 
furrow  now  appears,  nearer  the  northern  pole  ;  this,  there- 
fore, we  may  compare  to  the  arctic  circle.  The  result  of 
this  is  that  we  find  24  cleavage-cells :  16  upper,  smaller 
and  darker,  and  8  lower,  larger  and  brighter  (Cr).  The 
latter,  however,  soon  separate  into  16,  for  a  third  parallel 
circle  appears  in  the  southern  hemisphere  ;  there  are,  there- 
fore, 32  cells  in  all  (Fig.  31,  H).  Eight  new  meridian- 
furrows  now  arise  at  the  northern  pole,  and,  first  cutting 
the  upper,  darker,  cellular  circle,  afterwards  intersect  the 
lower,  southern  circle,  and  finally  reach  the  southern 
pole.  We  thus  find  stages  in  which  there  are  successively 
40,  48,  56,  and  finally,  64  cells  (/,  K}.  The  inequality 
between  the  two  hemispheres  constantly  becomes  greater. 
While  the  inert  southern  hemisphere,  for  a  long  time,  does 
not  add  to  its  32  cells,  the  vigorous  northern  half  of  the 
globe  furrows  itself  twice  successively,  and  thus  parts  into 
64,  and  then  into  128  cells  (Fig.  31,  L,  M).  In  the  stage 
in  which  we  now  see  the  egg,  there  are,  therefore,  128 
small  cells  on  the  surface  of  the  upper,  darker  half  of  the 
egg-sphere,  and  only  32 .  cells  in  the  lower,  brighter  half : 
LOO  cleavage-cells  in  all.  The  inequality  between  the  two 
hemispheres  increases  yet  further;  and  while  the  northern 


REPEATED   DIVISION   OF   CELLS.  2O5 

hemisphere  parts  into  a  very  large  number  of  small  cells, 
the  southern  -hemisphere  consists  of  a  much  smaller  number 
of  larger  cells.  Finally,  they  almost  entirely  overgrow 
the  surface  of  the  spherical  egg;  and  it  is  only  at  a  small 
circular  point  in  the  middle  of  the  lower  hemisphere,  at  the 
south  pole,  that  the  inner,  larger,  and  brighter  cells  are 
visible.  This  white  space  at  the  southern  pole  corresponds, 
as  we  shall  presently  see,  to  the  primitive  mouth  of  the 
Gastrula.  The  whole  mass  of  inner,  larger,  and  brighter 
cells  (together  with  this  white  space  at  the  pole)  belongs 
to  the  entoderm,  or  intestinal  layer.  The  outer  envelope  of 
dark,  smaller  cells  forms  the  exoderm,  or  skin-layer. 

The  often  repeated  division  of  the  cells,  which  as 
cleavage  or  segmentation  is  plainly  traceable  on  the  surface 
of  the  egg-sphere,  is  not  confined  to  this  surface,  but  ex- 
tends to  the  whole  interior  of  the  ball  of  the  egg.  The 
cells  also  segment  in  strata,  which  approximately  corre- 
spond to  concentric  strata  of  the  sphere ;  this  process  ad- 
vances more  quickly  in  the  upper  than  in  the  lower  half. 
A  large  cavity,  filled  with  liquid  forms,  has  in  the  mean 
time  arisen,  in  the  interior  of  the  egg-sphere ;  this  is  the 
cleavage-cavity  (a,  drawings  of  sections  in  Plate  II.  Fig. 
8-11).  The  first  trace  of  this  cavity  makes  its  appearance 
in  the  middle  of  the  upper  hemisphere,  at  the  point  at 
which  the  three  first  cleavage-planes,  which  are  at  right 
angles  to  one  another,  intersect  (Plate  II.  Fig.  8  s).  During 
the  progress  of  cleavage,  this  hollow  extends  significantly, 
and  afterwards  assumes  an  almost  hemispherical  form  (Fig. 
32  F;  Plate  II.  Fig.  9  s,  10  s).  The  arched  roof  of  this 
hemispherical  cleavage-cavity  is  formed  by  the  smaller, 
darker-coloured  cells  of  the  skin-layer,  or  exoderm  (Fig, 


2O6 


THE   EVOLUTION   OF   MAX. 


32,  D) ;  on  the  other  hand,  the  flat  floor  of  the  cavity  is 
composed  of  the  larger,  whiter-coloured  cells  of  the  intes- 
tinal layer,  or  eiitoderm  (Fig.  32  z\ 


FIG.  32-35. — Four  longitudinal  sections  of  the  segmented  egg  of  a  Toad, 
in  four  successive  stages  of  evolution.  In  all,  the  letters  indicate  the  same 
parts :  F,  cleavage-cavity ;  D,  the  roof  of  this  cavity ;  R,  dorsal  half  of 
the  germ ;  B,  intestinal  half ;  P,  the  yelk-plug  (white  circular  space  at  the 
lower  pole)  ;  z,  yelk-cells  of  the  entoderm  (the  gland-germ  of  Remak) ; 
N,  primitive  intestinal  cavity  (protogaster,  or  Rusconi's  nutritive  cavity). 
The  primitive  mouth  is  filled  up  by  the  yelk-plug  (P)  ;  s,  boundary  between 
the  primitive  intestinal  cavity  (N)  and  the  cleavage-cavity  (JF)  ;  fc,  k',  section 
through  the  swollen  circular  lip  or  edge  of  the  primitive  mouth  (the  so- 
called  anus  of  Rusconi).  The  dotted  line  between  fe  and  V  indicates  the 
former  connection  between  the  yelk-plug  (P)  and  the  central  mass  of  yelk- 
cells  (2).  In  Fig.  35  the  egg  has  turned  round  90°,  so  that  the  dorsal  half 
of  the  germ  (R)  is  seen  above  ;  the  intestinal  half  (B)  is  now  turned  down- 
ward. (After  Strieker.) 


AMPHIBIAN   GASTRULA.  2O7 

A  second  cavity,  narrower  but  larger,  now  arises,  owing 
fco  an  inversion  of  the  lower  pole,  and  to  a  separation  in 
the  white  entoderm-cells  next  to  the  cleavage-cavity  (Fig 
32-35,  N}.  This  is  the  primitive  intestinal  cavity  or 
stomach-cavity  of  the  Gastrula,  the  Protogaster.  It  was 
first  observed  by  Rusconi  in  the  eggs  of  Amphibia,  and 
is  accordingly  called  Rusconi's  "nutritive  cavity."  In  the 
longitudinal  section  (Fig.  33)  it  appears  bent  and  sickle- 
shaped,  and  extends  from  the  south  pole  nearly  to  the 
north,  for  it  folds  a  portion  of  the  inner  intestinal  cells 
inward  and  upward — between  the  cleavage-cavity  (F)  and 
the  dorsal  skin  (R).  The  primitive  intestinal  cavity  is  so 
narrow  at  first  because  the  greater  part  of  it  is  filled  up 
with  the  yelk-cells  of  the  entoderm.  The  latter  also  plug 
up  the  entire  wide  opening  of  the  primitive  mouth,  and 
there  form  the  so-called  yelk-plug,  which  appears  from  the 
outside  as  the  white,  circular  spot  at  the  south  pole  (P). 
Round  this  yelk-plug  the  skin-layer  thickens,  swells,  and 
forms  the  lip  of  the  primitive  mouth  (the  properistoma, 
Fig.  35  k,  &').  Presently  the  primitive  intestinal  cavity  (N*) 
extends  gradually  at  the  cost  of  the  cleavage-cavity  (F) ; 
and,  finally,  the  latter  entirely  disappears.  A  thin  partition 
(Fig.  34,  a)  alone  separates  the  two  cavities.  That  portion 
of  the  germ  in  which  the  primitive  intestinal  cavity  de- 
velops, afterwards  becomes  the  dorsal  surface  (It).  The 
cleavage-cavity  lies  in  the  anterior,  the  yelk-plug  in  the 
posterior  part  of  the  body.68 

When  the  primitive  intestine  is  complete,  the  Fiog- 
embryo  has  reached  the  Gastrula  stage  (Plate  II.  Fig.  11). 
But  it  is  evident  that  this  kenogenetic  amphibian  Gastrula 
differs  greatly  from  the  genuine  palingenetic  Gastrula,  which 


2O8  THE   EVOLUTION   OF   MAN. 

we  saw  before  (Fig.  23-29).  In  the  latter,  the  Bell-gastrula 
(Archigastrula),  the  body  has  but  one  axis.  The  primitive 
intestine  is  empty,  and  the  opening  of  the  primitive  mouth 
is  wide.  The  skin-layer  and  the  intestinal  layer  consist 
each  of  a  single  cell  stratum.  The  two  lie  close  together, 
for  the  cleavage-cavity  has  entirely  disappeared  during  the 
process  of  unfolding.  The  amphibian  Hood-gastrula  (Am- 
phigastrula)  is  entirely  different  (Fig.  32-35;  Plate  II. 
Fig.  11).  In  this  the  cleavage-cavity  (F)  continues  for  a 
considerable  time  side  by  side  with  the  primitive  intestinal 
cavity  (N).  Yelk-cells  fill  the  greater  part  of  the  latter; 
and  they  also  fill  the  primitive  mouth  (yelk-plug,  P).  Both 
the  intestinal  layer  (z)  and  the  skin-layer  (a)  consist  of 
several  strata  of  cells.  Finally,  the  general  outline  of  the 
entire  Gastrula,  instead  of  having  only  one  axis,  has  three ; 
for  the  three  axes  which  characterize  the  bilateral  body 
of  the  higher  animals,  are  indicated  by  the  eccentric  evo- 
lution of  the  primitive  intestinal  cavity. 

In  the  evolution  of  the  Hood-gastrula  (Amphiga'strula) 
we  are  unable  to  distinguish  sharply  between  the  different 
epochs,  which,  marked  by  the  mulberry -germ  and  the  germ- 
vesicle,  we  saw  followed  each  other  in  the  case  of  the  Bell- 
gastrula  (Archigastrula).  The  Horula-stage  (Plate  II.  Fig. 
9)  is  as  indistinctly  separated  from  the  Blastu la-stage 
(Fig.  10),  as  the  latter  is  from  the  Gastrula  (Fig.  11),  But 
in  spite  of  this,  we  shall  not  have  much  difficulty  in  retra- 
cing the  whole  kenogenetic  or  vitiated  course  of  evolution 
of  this  amphibian  Amphigastrula  to  the  genuine,  palm- 
genetic  origin  of  the  Archigastrula  of  the  Amphioxus. 

It  LS  far  harder  to  do  this  in  the  case  of  Mammals, 
although  the  course  of  egg-cleavage  and  gastrulation  in 


DEVELOPMENT   OF   MAMMALIAN   EMBRYO.  2Og 

these  is,  on  the  whole,  very  similar  to  that  of  Amphibia. 
Until  recently  the  growth  of  the  mammalian  embryo  was 
entirely  wrongly  explained ;  and  it  is  only  lately  (1875) 
that  Van  Beneden,  whose  views  we  adopt  here,  pointed  out 
its  real  significance.69  His  studies  were  directed  towards 
the  embryo  of  the  Rabbit,  an  animal  in  connection  with 
which  Bischoff  first  discovered  the  history  of  the  mamma- 
lian germ.  As  the  Rabbit  in  common  with  Man  belongs  to 
the  group  of  disco-placental  Mammals,  as  this  Rodent 
develops  entirely  in  the  same  way  as  does  Man,  and  as  even 
at  a  later  stage  of  evolution  the  embryos  of  Man  and  of  the 
babbit  are  hardly  distinguishable  (cf.  Plate  VII.  Fig. 
K,  M),  there  is  not  the  slightest  reason  to  doubt  that  the 
egg-cleavage  and  gastrulation  of  the  two  are  similar. 

When  the  fertilization  of  the  egg  of  the  Rabbit  is  com- 
plete, and  the  elaboration  of  the  parent-kernel  has  trans- 
formed the  Monerula  (Fig.  36)  into  the  parent-cell,  or  cytula 
(Fig.  37),  the  latter  (the  cytula)  separates  into  the  two  first 
cleavage-cells  (Fig.  38).  In  this  process  the  parent-kernel 
first  becomes  fusiform  and  divides  into  two  kernels  (the 
two  first  cleavage-kernels).  These  repel  each  other  and  the 
two  move  apart.  After  this  the  protoplasm  of  the  parent- 
cell,  attracted  by  the  two  kernels,  parts  into  two  halves, 
each  of  which  assumes  a  globular  form.  They  afterwards 
change  from  this  globular  to  an  ellipsoid  form  (Fig.  38). 
These  two  cleavage-cells  are  not,  as  was  formerly  believed, 
of  the  same  size  and  significance.  The  one  is  larger, 
brighter,  and  more  transparent  than  the  other.  Again,  the 
smaller  cleavage-cell  takes  a  much  deeper  colour  from  car- 
mine, osmium,  etc.,  than  does  the  larger.  The  two  cells 
thus  already  betray  theii  relations  to  the  two  primitive 


2IO 


THE   EVOLUTION   OF   MAN. 


FIG.  36.— Monernla  of  a  Mammal  (Rabbit).  The  fertilized  egg-cell  after 
loss  of  the  germ-vesicle  is  a  simple  ball  of  protoplasm  (d).  The  outer 
envelope  of  this  is  formed  by  the  modified  zona  pellucida  (z)  and  by  a  mucous 
layer  (h),  which  is  deposited  on  the  outside  of  the  zona.  A  few  sperm-cells 
are  still  visible  in  this  mucous  layer  (s). 

FIG.  37.— Parent-cell,  or  cytula,  of  a  Mammal  (Rabbit) :  k,  parent -kernel, 
or  nucleus;  n,  nucleolus;  p,  protoplasm  of  the  parent-cell;  z,  modified 
zona  pellucida  ;  h,  external  albuminous  envelope  ;  s,  sperm-cells. 

FIG.  38. — Commencement  of  cleavage  in  the  mammalian  egg  (Rabbit). 

The  parent-cell  has  separated  into  two  differing  cells;  the  brighter, 
mother-cell  of  the  skin-layer  (e),  and  the  darker,  mother-cell  of  the  in- 
testinal layer  (j)  :  z,  zona  pellucida;  h,  external  albuminous  envelope; 
s,  dead  sperm-cells. 


EXODERM  AND  EXTODERM  CELLS.          211 

germ-layers.  The  brighter  and  harder  cleavage-cell  (Fig. 
38,  e)  is  the  mother-cell  of  the  exoderm ;  the  darker  and 
softer  cleavage-cell  (Fig.  38,  i)  is  the  mother-cell  of  the 
entoderm.  All  the  cells  of  the  outer  germ -layer,  the  skin- 
layer,  are  produced  from  the  exoderm  mother-cell  (Fig. 
38,  e;  Plate  II.  Fig.  13,  e).  In  the  same  way  the  whole  of 
the  cells  of  the  inner  germ-layer,  the  intestinal  layer, 
descend  from  the  entoderm  mother-cell  (Fig.  38,  i\  Plate 
II.  Fig.  13,  i).  This  interesting  relation,  which  we  thus  see 
in  the  mammalian  germ,  is  yet  more  pronounced  in  the 
germs  of  many  lower  animals.  In  many  Worms,  for 
example,  at  the  beginning  of  cleavage,  the  parent-cell 
parts  into  two  cleavage-cells  of  very  dissimilar  size  and 
chemical  qualities.  In  such  cases  the  mother-cell  of  the 
exoderm  is  often  very  many  times  smaller  than  the  ento- 
derm mother-cell,  which  contains  a  large  store  of  nutritive 

yelk. 

The  two  first  cleavage-cells  of  the  Mammal,  which  are 
to  be  regarded  as  the  mother-cells  of  the  two  primary  germ- 
layers,  now  contemporaneously  separate  into  two  cells  (Fig. 
39 ;  Plate  II.  Fig.  14).  These  four  cleavage-cells  usually  lie 
in  two  different  planes,  perpendicular  to  each  other;  more 
rarely  in  one  plane.  The  two  larger  and  brighter  cells 
(Fig.  39,  e),  the  descendants  in  the  first  generation  of  the 
exoderm  mother-cell,  if  placed  in  carmine,  colour  much 
more  deeply  than  do  the  two  smaller  and  darker  cells,  the 
descendants  of  the  entoderm  mother-cells  (Fig.  39,  i).  The 
line  which  connects  the  central  points  of  the  two  latter 
cleavage-globules  is  usually  perpendicular  to  that  which 
connects  the  central  points  of  the  two  latter.  Presently 
each  of  these  four  cells  again  divides  into  two  similar  cells ; 


212 


THE    EVOLUTION   OF   MAN. 


we  therefore  find  that  there  are  now  eight  cleavage-cells, 
the  descendants  in  the  third  generation  of  the  parent-cell 
(Fig.  40).  Four  larger,  brighter,  and  firmer  cells  lie  in  one 
plane;  the  descendants  in  the  second  generation  of  the 
exoderm  mother-cell.  Four  smaller,  darker,  and  softer  cells 
lie  in  a  second  plane,  perpendicular  to  the  former ;  the 
descendants  in  the  second  generation  of  the  entoderm 
mother-cell.  If  we  connect  the  central  points  of  the  oppo- 
site cleavage-cells  of  one  plane,  two  and  two,  by  straight 
lines,  these  lines  meet  each  other  at  right  angles.  But  the 
four  connecting  lines  of  the  two  parallel  planes  together 
intersect  at  an  angle  of  forty -five  degrees  (Fig.  40). 


FTG.  39.— The  four  first  cleavage-cells  of  a  Mammal  (Babbit)  :  e,  the 
two  exoderm-cells  (larger  and  brighter) ;  i,  the  two  entoderm-cells  (smaller 
and  darker) ;  z,  zona  pellucida  ;  h,  outer  albuminous  envelope. 

FIG.  40. — Egg  of  Mammal  (Rabbit),  with  eight  cleavage-cells:  e,  four 
exoderm-cells  (larger  and  brighter)  ;  i,  four  entoderin-cells  (smaller  and 
darker) ;  z,  zona  pellucida ;  h,  outer  albuminous  covering. 

Now,  however,  the  eight  cleavage-cells  alter  their 
original  position,  and  lose  their  globular  form.  One  of  the 


FORMATION    OF   GASTKULA. 


213 


four  exoderm-cells  makes  its  way  into  the  middle  of  the 
cell-mass,  and,  together  with  its  three  fellows,  forms  a  pyra- 
mid (or  tetrahedron).  The  four  exoderm-cells  arrange 
themselves  in  the  form  of  a  cap  over  the  point  of  this 
pyramid  (Plate  II.  Fig.  15).  This  is  the  beginning  of  a 
germinal  process  which  must  be  regarded  as  a  shortened 
and  vitiated  repetition  of  the  inversion  of  the  germ-mem- 
brane vesicle,  and  which  results  in  the  formation  of  a  Gas- 
trula.  From  this  time  the  further  cleavage  of  the  mam- 
malian egg  adheres  to  a  rhythm  which  is  most  essentially 
similar  to  that  of  the  Frog's  egg.  While  in  the  original 
(or  primordial;  egg- cleavage,  the  rhythm  advances  in  regular 
geometrical  progression  (2,  4,  8,  16,  32,  64,  128,  and  so  on) ; 
in  the  modified  progression  of  the  mammalian  egg,  the 
sequence  of  numbers  is  the  same  as  that  of  the  amphibian 
egg  :  2,  4,  8,  12, 16,  24,  32, 48,  64,  96,  160,  etc.  (Cf  Table  V.) 


FIG.  41.  —  Gastrula  of  a 
Mammal  (Amphigastrula  of  a 
Rabbit),  in  longitudinal  section 
through,  the  axis  :  e,  exoderm- 
cells  (64  brighter  and  smaller)  ; 
i,  entoderm-cells  (32  darker  and 
larger)  ;  d,  central  entoderm- 
cells,  filling  up  the  primitive  in- 
testinal cavity  ;  o,  external  ento- 
derm-cells, plugging  the  primi- 
tive mouth-opening  (yelk-plug 
in  the  "  auus  of  Eusconi  "). 


This  depends  on  the  fact  that  from  this  time  the  more 
vigorous  exoderm-cells  increase  at  a  quicker  rate  than  the 
more  inert  entoderm-cells.  The  latter  always  remain 
behind  the  former,  and  are  overgrown  by  them.  This  pro- 


214  THE   EVOLUTION    OF  MAN. 

cess  in  which  the  inner  intestinal  layer  cells  are  overgrown, 
is  really  nothing  but  the  inversion  of  the  vegetative  hemi- 
sphere into  the  animal  hemisphere  of  the  germ-vesicle ;  i.e. 
the  formation  of  a  Gastrula  (Fig.  41).69 

Next,  therefore,  follows  a  stage  in  which  the  mamma- 
lian germ  consists  of  12  cleavage-cells;  4  darker  entoderm- 
cells  form  a  three-sided  pyramid  which  is  covered  by  a  cap 
of  12  lighter  exoderm-cells  (Plate  II.  Fig.  15  in  section). 
The  next  stage,  in  which  there  are  16  cleavage-cells,  is  seen 
to  consist  of  4  entoderm-cells  in  the  interior,  4  other  outer 
and  lower  entoderm-cells ;  while  the  8  exoderm-cells,  in  the 
form  of  a  hemispherical  cap,  cover  the  upper  half  of  the 
germ.  This  cap  of  exoderm-cells,  which  increase  in  number 
from  8  to  16,  continues  to  overgrow  the  inner  cell  mass ;  of 
the  8  entoderm-cells,  3,  4,  or  5  lie  in  the  centre  of  the  germ, 
and  the  rest  at  the  base  of  the  globular  germ  (Plate  II. 
Fig.  16).  This  24-celled  stage  is  followed  by  one  in  which 
there  are  32,  for  the  8  entoderm-cells  also  double  their 
number.  This  is  afterwards  succeeded  by  germ-forms  in 
which  there  are  48  cleavage-cells  (32  exoderm  and  16  ento- 
derm);  64  cleavage-cells  (32  skin-layer  and  32  intestinal 
layer) ;  96  cleavage-cells  (64  exoderm  and  32  entoderm), 
and  so  on. 

When  the  mammalian  embryo  has  acquired  96  cleavage- 
cells,  a  stage  which,  in  the  case  of  the  Rabbit,  is  reached 
in  about  the  70th  hour  after  fertilization,  the  charac- 
teristic form  of  the  Hood-gastrula  (Amphigastrula)  becomes 
plainly  visible  (Fig.  41 ;  cf.  Plate  II.  Fig.  17  in  section). 
The  globular  embryo  consists  of  a  central  mass  of  32  soft, 
roundish,  dark  granular  entoderm-cells,  which,  by  mutual 
pressure,  are  flattened  multilaterally,  and  which  assume 


MAMMALIAN   HOOD-GASTRULA,  215 

a  aark  brown  colour  when  treated  with  osmic  acid  (Fig. 
41,  i).  This  dark  central  cellular  mass  is  surrounded  by 
a  brighter  globular  membrane,  composed  of  64  smaller  cube- 
shaped  and  finely  granulated  exoderm-cells,  which  lie  side 
by  side  in  a  single  layer,  and  take  up  very  little  colour  from 
osmic  acid  (Fig.  41,  e).  The  exoderm-membrane  is  broken 
only  at  one  single  point,  when  1,  2,  or  3  entoderm-cells 
pierce  to  the  surface.  The  latter  form  the  yelk-plug 
which  entirely  occupies  the  primitive  mouth  of  the  Gastrula 
(o).  The  central  primitive  intestinal  cavity  is  filled  by 
entoderm-cells  (Plate  II.  Fig.  17).  The  single  axis  of  the 
outline  of  the  mammalian  Gastrula  is  thus  clearly  indi- 
cated.69 

Although  the  unequal  egg-cleavage  and  gastrulation  of 
Mammals  and  Amphibia  present  various  peculiarities,  it 
is  comparatively  easy  to  trace  these  processes  back  to  the 
egg-cleavage  and  gastrulation  of  the  lowest  Vertebrate,  the 
Amphioxus,  which  is  entirely  similar  to  the  form  of  cleav- 
age carefully  examined  by  us  in  the  case  of  the  Coral.  (Cf. 
Fig.  22  and  28.)  All  these  and  many  other  classes  of 
animals  agree  in  that,  in  their  egg-cleavage,  the  whole  egg 
parts,  by  repeated  division,  into  a  large  number  of  cells. 
All  such  animal  eggs  have  long  been  called  holoblastic,  a 
name  given  them  by  Remak,  because  in  them  the  cleavage 
into  cells  extends  to  the  whole  mass ;  or,  in  other  words,  is 
total  (Plate  II.). 

In  very  many  other  classes  of  animals  this  is,  however, 
not  the  case ;  for  instance,  among  Vertebrates,  in  Birds,  Rep- 
tiles, and  most  Fishes ;  among  Articulated  animals  (Arthro- 
poda),  in  Insects,  most  Spiders  and  Crabs;  among  Soft- 
bodied  animals  (Mollusca),  in  Cephalopods  or  Cuttle-fishes, 


2l6  THE   EVOLUTION   OF  MAN. 

In  all  these  animals,  both  the  ripe  egg-cell,  and  the  parent- 
cell,  into  which  fertilization  transforms  this  egg-cell,  consist 
of  two  quite  distinct  and  separate  parts,  which  are  distin- 
guished respectively  as  the  formative  yelk  and  the  nutritive 
jelk.  The  formative  yelk  (vitellus  formativus,  or  morpho- 
lecithus)  is  the  nucleated  egg-cell,  capable  of  evolution,  which 
divides  in  the  process  of  cleavage,  and  produces  the  nu- 
merous cells  which  constitute  the  embryo.  The  nutritive 
yelk  (vitellus  nutritivus,  or  tropholecithus),  on  the  other 
hand,  is  a  mere  appendage  of  the  true  egg-cell,  and  contains 
hoarded  food -substance  (albumen,  fat,  etc.);  so  that  it  forms 
a  sort  of  storehouse  for  the  embryo  in  the  course  of  its 
evolution.  The  embryo  absorbs  a  quantity  of  nutritive 
matter  from  this  storehouse,  and  finally  entirely  consumes  it. 
Indirectly,  therefore,  the  nutritive  yelk  is  of  great  import- 
ance in  germination.  Directly,  however,  it  takes  no  share 
in  the  process,  for  it  is  not  concerned  in  the  cleavage,  and 
is  not  cellular.  Sometimes  the  nutritive  yelk  is  smaller, 
sometimes  larger;  generally  many  times  larger  than  the 
formative  yelk- ;  for  which  reason,  greater  importance  was 
formerly  attached  to  the  nutritive  than  to  the  formative 
yelk.  All  eggs  which  have  this  independent  nutritive  yelk, 
and  of  which,  therefore,  only  a  portion  undergoes  cleavage, 
are  called  meroblastic,  the  name  given  them  by  Kemak; 
their  cleavage  is  incomplete  or  partial  (Plate  III.). 

It  is  not  easy  correctly  to  apprehend  this  partial  egg- 
cleavage,  and  the  peculiar  form  of  Gastrula  which  results 
from  it;  and  it  was  only  quite  recently  that  comparative 
research  enabled  me  to  remove  this  difficulty,  and  to  retrace 
this  kenogenetic  form  of  cleavage  and  gastrulation  to  the 
original,  palingenetic  form.  The  sea  eggs  of  one  of  the 


DEVELOPMENT   OF    F1SH-GASTRULA.  2 17 

Osseous  Fishes  (Teleostei),  the  evolution  of  which  I  studied 
at  Ajaccio,  in  Corsica,  in  1875,  were  of  the  greatest  service 
to  me  in  this  respect  (Plate  III.  Fig.  18-24).  I  found  these, 
massed  together  in  lumps  of  jelly,  floating  on  the  surface  of 
the  sea ;  and  as  the  tiny  eggs  were  quite  transparent,  I  was 
easily  able  to  watch  each  stage  in  the  evolution  of  the 
germ.70  These  eggs,  probably  those  of  a  cod-fish  of  the 
Gaddoid  family,  but  perhaps  of  a  Cottoid,  are  colourless 
globules,  as  transparent  as  glass,  and  of  rather  more  than  half 
a  millimetre  in  diameter  (0'64 — 0'66  mm.).  Within  a  thin, 
structureless  but  firm  egg-membrane  (chorion,  Fig.  42,  c)  lies 

FIG.  42. — Egg  of  an  oceanic  Osseous 
Fish :  p,  protoplasm  of  the  parent-cell ;  fe, 
kernel  of  parent-cell ;  n,  clear  albumin- 
ous ball  of  nutritive  yelk;  /,  fat-globule 
of  the  latter ;  c,  external  egg-membrane, 
or  chorion. 

a  large  albuminous  ball,  which 
is  quite  transparent  and  as  clear 
as  water  (71).  At  both  poles  of 
the  axis  of.  this  ball  there  is  a 

groove-like  indentation.  In  the  groove  at  the  upper  pole, 
which,  in  the  floating  egg,  is  turned  downwards,  lies  a 
simple,  lentil-shaped  cell,  containing  a  kernel  (Fig.  42,  p). 
In  the  unfertilized  egg,  this  is  the  original  egg-cell ;  after 
fertilization  it  is  the  parent-cell.  In  the  interval  between 
these  two  nucleated  stages  there  is  probably  a  non- 
nucleated  condition,  representing  the  Monerula.  At  the 
opposite  pole  of  the  egg,  in  the  lower  groove,  lies  a  simple, 
clear  fat-globule  (/).  This  small  fat-globule  and  the  large 
albuminous  globule  together  form  the  nutritive  yelk.  The 


2l8  THE   EVOLUTION   OF   MAN. 

small  cell  alone  is  the  formative  yelk,  and  is  the  only  pan 
concerned  in  the  cleavage  process,  which  does  not  extend 
to  the  nutritive  yelk.70. 

The  cleavage  of  the  parent-cell,  or  the  formative  yelk, 
proceeds  entirely  independently  of  the  nutritive  yelk,  and 
in  quiet,  regular,  geometric  progression.  (Of.  Plate  III.  Fig. 
18-24.)  Only  the  formative  yelk,  with  the  contiguous 
portion  of  the  nutritive  yelk  (ri),  is  represented  in  the 
perpendicular  section  (through  a  meridian-plane) ;  the 
greater  part  of  the  nutritive  yelk  and  the  egg-membrane 
is  therefore  omitted.  The  parent-cell  (Fig.  18),  first  sepa- 
rates into  two  similar  cleavage-cells  (Fig.  19).  By  repeated 
division,  this  gives  rise  to  4,  then  8,  then  16  cells  (Fig.  20). 
By  continued  contemporaneous  division,  32,  and  then  64 
cells  originate  from  these  ;  and  so  the  process  goes  on.  All 
these  cleavage-cells  are  alike  in  size  and  character.  At  last 
they  form  a  lentil-shaped  mass  of  closely  layered  cells  (Plate 
III.  Fig.  21).  This  entirely  corresponds  to  the  globular 
mulberry -germ  of  the  primordial  cleavage-process  (Morula, 
Plate  II.  Fig.  3).  The  cells  of  this  lentil-shaped  mulberry- 
germ  now  move  off  in  a  peculiar  centrifugal  direction, 
so  that  the  mulberry-germ  changes  into  a  vesicular  germ 
(Blastula,  Plate  III.  Fig.  22).  The  ordinary  lentil  be- 
comes a  disc,  in  the  shape  of  a  watch-glass,  with  thickened 
edges.  Just  as  a  watch-glass  lies  upon  a  watch,  this  con- 
vex cellular  disc  lies  on  the  upper,  more  slightly  arched, 
pole  surface  of  the  nutritive  yelk.  Meanwhile,  liquid  has 
collected  between  the  disc  and  the  surface  of  the  nutritive 
yelk,  so  that  a  low  circular  cavity  has  been  formed  (Fig.  22,  s). 
This  is  the  cleavage-cavity,  and  corresponds  to  the  cleavage- 
cavity  in  the  centre  of  the  palingentic  Blastula  (Plate  II 


THE    'DISC-GASTWLA.  2  IQ 

Fig.  4).  The  slightly  arched  floor  of  this  low  cleavage- 
cavity  is  formed  of  nutritive  yelk  (n) ;  the  more  arched  roof 
is  of  Blastula-cells.  In  fact,  the  embryonic  Fish  is  now  a 
vesicle  with  an  eccentric  cavity,  as  was  the  Blastula  of  the 
Frog  (Plate  II.  Fig.  10). 

The  important  process  of  inversion,  resulting  in  gastru- 
lation,  now  takes  place.  In  consequence  of  a  further  re- 
moval, or  wandering,  of  the  blastula-cells,  and  of  a  further 
increase  in  their  number,  the  thickened  edges  of  the  cellular 
disc,  which  lie  on  the  nutritive  yelk,  grow  toward  each 
other  in  a  centripetal  direction,  and  toward  the  centre  of 
the  cleavage-cavity  (Fig.  23),  at  which  point  they  finally 
unite.  The  whole  cell-mass  now  forms  a  small  flat  sac  lying 
on  the  top  of  the  nutritive  yelk.  The  cavity  of  this  sac, 
the  cleavage -cavity,  soon,  however,  disappears,  because  the 
whole  upper  surface  of  the  lower  wall  of  the  sac  attaches 
itself  closely  to  the  whole  lower  surface  of  the  upper  wall 
(Fig.  24).  This  completes  the  gastrulation  of  this  Fish. 


FIG.  43. — Disc-gastrula  (Disco-gas- 
rula)  of  an  Osseous  Fish  :  e,  exoderm  ; 
,  entoderm  ;  w,  swollen  edge,  or  primi- 
ive  mouth-edge ;  n,  albuminous  ball 
f  nutritive  yelk  ;  /,  fat-globule  with- 
11  the  latter ;  c,  outer  egg-membrane 
chorion) ;  d,  boundary  between  ento- 
_>rm  and  exoderm  (former  site  of  the 
cleavage-cavity). 


In  order  to  distinguish  this  third  important  form  of 
Gastrula  from  the  two  previously  mentioned,  we  will  call  it 
the  Disc-gastrula  (Disco-gastrula,  Fig.  43).  The  cell-mass  of 
this  Gastrula  forms  a  thin,  circular  disc.  The  lower  concave 


22O  THE   EVOLUTION   OF   MAN. 

surface  of  this  disc  lies  immediately  on  the  upper,  convex 
surface  of  the  nutritive  yelk  (11).  On  the  other  hand,  the 
outer  surface  of  the  disc  is  convex  as  in  a  Shark.  If  we 
make  a  perpendicular  section  through  a  meridian-plane  of 
the  globe-shaped  egg,  we  shall  find  that  it  is  composed  of 
several  layers  of  cells  (in  this  particular  case  there  are  four) 
(Plate  III.  Fig.  24).  Immediately  above  the  nutritive  yelk 
lies  a  single  layer  of  larger  cells  (Fig.  24,  i\  which  are 
characterized  by  a  softer,  less  transparent,  and  more  coarsely 
granulated  protoplasm,  and  which  take  up  a  dark  red  colour 
from  carmine.  These  form  the  intestinal  layer,  or  entoderm, 
which  arises  by  the  ingrowth  of  the  edges  of  the  disc 
(infolded  germ-layer).  The  three  outer  layers,  lying  on  top 
of  this  lower  layer,  form  the  skin-layer,  or  exoderm  (Fig.  24,  e). 
They  consist  of  smaller  cells  which  take  only  a  slight  colour 
from  carmine ;  their  protoplasm  is  firmer,  more  transparent, 
and  more  finely  granulated.  At  the  thickened  edges  of  the 
gastrula,  the  primitive  mouth-edge  (properistoma),  the 
entoderm,  and  the  exoderm  pass  into  each  other  without 
clear  limits  (Fig  43,  w). 

It  is  evident  that  the  most  important  peculiarities  which 
distinguish  the  Disc-gastrula  from  the  two  typical  Gastrula- 
forms  which  we  before  examined,  are  due  to  the  large  nutri- 
tive yelk.  This  takes  no  part  in  the  cleavage,  and  from  the 
first  occupies  the  whole  primitive  intestinal  cavity,  while  at 
the  same  time  it  extends  far  beyond  the  mouth-opening  of 
the  latter.  If  we  imagine  the  original  Bell-gastrula  (Archi- 
gastrula,  Fig.  23-29)  attempting  to  swallow  a  globe  of 
nutritive  matter  far  larger  than  itself,  in  the  attempt  the 
Gastrula  will  be  spread  out  in  the  form  of  a  disc  on  the 
nutritive  matter,  much  in  the  same  way  as  in  the  Disc- 


THE   NUTRITIVE   YELK.  221 

gastrula  (Disco-gastrula,  Fig.  43).  We  may  therefore  infer 
that  the  latter  is  directly,  or  through  the  intermediate  stage 
of  the  Hood-gastrula,  descended  from  the  original  Bell- 
gastrula.  It  arose  phylogenetically  owing  to  the  fact  that 
a  store  of  nutritive  matter  collected  at  one  pole  of  the  egg, 
and  thus  formed  a  nutritive  yelk  distinct  from  the  forma- 
tive yelk.  Yet,  notwithstanding  this,  the  Gastrula  in  this, 
as  in  the  former  cases,  was  originated  by  an  inversion  or 
invagination  of  the  Blastula.  We  may,  therefore,  also  refer 
this  kenogenetic  form  of  discoidal  cleavage  (segmentatio 
discoidalis)  to  the  original  and  palingenetic  form. 

Although  it  is  thus  tolerably  easy  and  safe  to  trace  back 
the  descent  of  the  small  egg  of  this  oceanic  Osseous  Fish,  yet, 
on  the  other  hand,  it  seems  hard  to  do  this  with  certainty 
in  the  case  of  larger  eggs,  such  as  occur  in  the  case  of  most 
other  Fishes,  and  in  the  case  of  all  Reptiles  and  Birds.  In 
the  first  place,  the  nutritive  yelk  of  these  is  quite  dispro- 
portionately large ;  so  large,  indeed,  that  it  almost  causes 
the  formative  yelk  to  disappear.  And,  in  the  second  place, 
the  nutritive  yelk  contains  a  number  of  variously  formed 
constituent  parts,  which  are  known  as  the  yelk-granules, 
yelk-globules,  yelk-vesicles,  and  so  on.  These  definite  yelk- 
elements  have  often  even  been  explained  as  true  cells, 
and  it  has  been  quite  wrongly  assumed  that  a  portion 
of  the  body  of  the  embryo  is  found  in  them.71  This 
is  by  no  means  the  case.  The  nutritive  yelk,  what- 
ever its  size,  always  remains  a  lifeless  store  of  nutritive 
natter,  which,  in  the  process  of  germination,  is  taken  into 
the  intestine  during  its  development,  and  is  consumed  by 
the  embryo.  The  latter  develops  solely  from  the  living 
formative  yelk,  from  the  parent-cell.  This  is  equally  true 


222  THE   EVOLUTION   OF   MAN. 

of  the  small  Osseous-fish  which  we  have  been  examining, 
and  of  the  huge  eggs  of  the  Primitive  Fishes  (Selachii),  of 
Reptiles,  and  of  Birds. 

The  egg  of  the  Bird  is  specially  important  to  us,  for 
most  of  the  important  researches  into  the  evolution  of 
Vertebrates  have  been  founded  on  study  of  incubated  hen's 
eggs.  It  is  much  harder  to  procure  and  to  examine  mam- 
malian eggs  ;  for  which  very  practical  and  incidental  reason 
the  latter  has  been  more  rarely  accurately  studied.  On 
the  other  hand,  hen's  eggs  can  always  be  obtained  in 
any  quantity,  and  artificial  hatching  enables  us  accurately 
to  follow  every  stage  in  the  changes  undergone  by  the 
embryo  in  the  course  of  its  evolution.  As  we  have  seen, 
the  chief  difference  which  distinguishes  the  egg  of  the 
Bird  from  the  minute  egg  of  the  Mammal  is  the  very  con- 
siderable size  of  the  former,  which  is  due  to  the  accumula- 
tion of  a  very  large  mass  of  fatty  nutritive  yelk.  This  is 
the  yellow  mass  which,  daily  consumed  under  the  name  of 
yelk  of  egg,  is  collected  within  the  original  yelk  or  proto- 
plasm of  the  egg-cell.  In  order  to  obtain  a  correct  con- 
ception of  the  Bird's  egg,  the  nature  of  which  has  very 
frequently  been  misrepresented,  we  must  search  for  it  in 
its  earliest  condition,  and  follow  its  evolution  from  its 
beginning  in  the  ovary.  In  this  stage,  we  find  that  the 
original  egg  is  a  very  small,  naked,  and  simple  cell  with 
a  nucleus,  and  that  it  differs  neither  in  size  or  shape 
from  the  original  egg-cell  of  Mammalia  and  other  animals. 
(Of.  Fig.  10  E,  p.  134.)  As  in  all  Skulled-animals  (Craniota) 
the  original  egg-cell  or  primitive  egg  (protovum)  is  com- 
pletely covered  by  a  continuous  layer  of  smaller  cells,  as 
though  by  an  epithelium.  This  skin-coat,  or  epithelium,  is 


PROTOVUM   AND   METOVUM. 


223 


the  so-called  Graafian  follicle  ;  immediately  under  this  the 
structureless  yelk -membrane  is  secreted  by  the  egg-yelk. 

At  a  very  early  period  the  small  protovum  of  the  Bird 
begins  to  imbibe  a  mass  of  food-substance  through  the 
yelk-membrane,  and  to  elaborate  this  matter  into  the  so- 
called  "  yellow  yelk."  The  protovum  is  thus  transformed 
into  the  metovum  (after-egg),  which  is  many  times  larger 
than  the  protovum,  but  which,  nevertheless,  is  only  a  single, 
enormously  enlarged  cell.72  The  accumulation  of  the  large 
yellow-yelk  mass  within  the  ball  of  protoplasm  forces  the 
kernel  (vesicula  germinativa),  which  is  contained  in  the 
latter,  quite  to  the  upper  surface  of  the  yelk-mass.  Here 
the  kernel  (yesicula  germinativa)  is  surrounded  by  a  small 
quantity  of  protoplasm ;  and  these  two  together  form  the 
lentil-shaped  "  formative  yelk "  (Fig.  44,  6).  This  appears 
on  the  outside  of  the  yellow  yelk-mass,  at  a  particular 
point  of  the  upper  surface,  in  the  form  of  a  small,  white, 
circular  point ;  the  so-called  "tread,"  or  cicatricula.t  A 


FIG.  44. — A  mature  egg-cell  from  the 
ovary  of  a  Hen  (in  section).  The  yellow 
nutritive  yelk  is  composed  of  concentric 
layers  (c),  and  is  surrounded  by  a  thin  yelk- 
membrane  (a).  The  cell-kernel  (yesicula  germi- 
natira),  together  with  the  protoplasm  of  the 
egg-cell,  forms  the  formative  yelk  (6),  or  the 
tread.  The  white  yelk  (here  represented  as 
black)  passes  from  the  tread  to  the  yelk- 
cavity  ('I').  The  two  kinds  of  yelk  arc. 
however,  not  sharply  distinguished. 


thread-like  cord  of  white  nutritive  yelk  (cl),  which  contains 
no  particles  of  yellow  yelk,  and  is  softer  than  the  yellow 
nutritive  yelk,  passes  from  the  tread  directly  to  tho 


224  THE   EVOLUTION   OF   MAN. 

centre  of  the  yellow  yelk-mass,  and  there  forma  a  small 
central  ball  of  white  yelk  (Fig.  44,  d).  The  whole  mass  of 
this  white  yelk  is,  however,  not  sharply  divided  from  the 
yellow  yelk,  which  in  hardened  eggs  shows  a  slight  trace 
of  concentric  stratification  (Fig.  44,  c).  Just  as  in  this 
globular  egg  in  the  ovary,  so  also  in  the  hen's  egg  after 
it  has  been  laid;  when  the  egg-shell  is  opened  and  the 
yelk  taken  out,  a  small,  circular,  white  disc  is  seen  on 
the  upper  surface  of  the  latter.  This  disc  represents  the 
cicatricula,  or  tread.  This  small  white  germ-disc  is,  how- 
ever, far  advanced  in  development,  and  is,  in  fact,  the 
Gastrula  of  the  hen.  The  body  of  the  latter  proceeds 
entirely  from  this  Gastrula.  The  whole  mass  of  white  and 
yellow  yelk  is  entirely  without  share  in  the  formation  of  the 
Chick,  for  it  is  only  used  up  as  nutritive  matter  and  con- 
sumed as  food  by  the  embryo  in  the  course  of  its  evolution. 
The  transparent,  tough,  and  voluminous  mass  of  albumen, 
surrounding  the  yellow  yelk  of  the  Bird's  egg,  and  the  hard 
chalky  shell  of  the  egg,  are  formed  round  the  egg,  in  the 
oviduct,  after  it  is  already  fertilized. 

After  the  fertilization  of  the  egg  within  the  body  of  the 
parent  Bird  is  complete,  the  germ-vesicle  (vesicula  ger- 
minativa)  probably,  as  in  other  cases,  first  disappears  ;  and 
the  reconstruction  of  a  kernel  results  in  a  parent-cell 
(cytuld).  This  lentil-shaped  parent-cell  now  undergoes  a 
discoidal  cleavage  (segmentatio  discoidalis,  Fig.  45)  entirely 
similar  to  that  of  the  egg  of  the  Fish  (Plate  III.  Fig.  18-24). 
Two  similar  cleavage-cells  (A)  first  arise  from  the  'parent- 
cell.  These  part  into  4  (£),  into  8,  16  (0),  32,  64  cells,  and 
so  on.  As  before,  the  division  of  the  kernel- always  precedes 
the  division  of  the  cells.  The  planes  of  division  between 


CLEAVAGE   OF   THE   BIRDS    EGG. 


225 


the  cleavage-cells  appear  at  the  free  surface  of  the  "  tread  " 
as  "furrows."  The  two  first  furrows  are  at  right  angles 
to  each  other,  in  the  form  of  a  cross  (5). .  Two  new  furrows 
then  originate,  which  cut  the  former  two  at  an  angle  of  45°. 
Th3  tread,  which  is  changing  into  the  germ-disc,  now  forms 


FIG.  45. — Discoidal  cleavage  of  a  Bird's  egg  (diagrammatic,  about  ten 
times  enlarged).  Only  the  formative  yelk  (the  tread,  or  cicatricula,  is  repre- 
sented in  these  6  figures  (A-F~),  because  it  alone  is  affected  by  cleavage. 
The  much  larger  nutritive  yelk,  which  does  not  share  in  the  cleavage,  is 
omitted,  and  only  indicated  by  the  dark,  outer  ring.  A.  The  first  furrow 
separates  the  parent-cell  into  two  parts.  B.  These  two  first  cleavage-cells 
are  parted  by  a  second  furrow  (perpendicular  to  the  first)  into  four  cells. 
C.  16  cells  have  originated  from  the  4  cleavage-cells,  owing  to  the  fact  that 
between  the  first  two  bisecting  furrows,  two  other,  radial  furrows  have 
appeared,  and  that  the  central  portions  of  these  8  radial  segments  by 
a  furrow  running  round  the  centre.  D.  A  stage  with  16  radial  furrows  and 
about  4  concentric  ring-furrows.  E.  A  stage  with  61  radial  furrows  and 
about  6  ring-furrows.  F.  The  whole  tread  has  been  broken  up  into  a  heap 
of  small  cells  by  the  further  formation  of  radial  and  ring  furrows ;  the  whole 
now  forms  the  lentil-shaped  mulberry-germ  (Morula).  The  separation  of 
the  kernel  always  precedes  the  formation  of  the  furrows. 


226  THE   EVOLUTION    OF   MAN. 

an  eight-rayed  star.  A  circular  furrow  now  forms  round 
the  centre,  so  that  the  8  three-cornered  cleavage-cella 
become  16,  of  which  8  lie  in  the  middle,  surrounded  by 
8  others  ((7).  After  this,  new  furrows,  some  circular  and 
others  radiating  from  the  central  point,  succeed  each  other 
more  or  less  irregularly  (D,  N).  Finally  this  cleavage- 
process,  like  the  others,  results  in  the  formation  of  small 
cells  of  like  character.73  In  this  case  also,  the  cleavage- 
cells  form  a  circular  lentil-shaped  disc,  which  represents  the 
mulberry-germ,  and  lies  embedded  in  a  slight  deepening  in 
the  white  yelk  (Fig.  46,  in  perpendicular  section).  The 
Morula  in  the  case  of  the  Hen's  egg  is,  however,  thinner  and 
flatter  than  that  of  the  egg  of  the  Osseous  Fish  (Plate  III. 
Fig.  21). 

In  the  Hen's  egg,  just  as  in  that  of  the  Osseous  Fish,  a 
kenogenetic  germ-vesicle,  or  Blastula,  now  arises  (Fig.  47). 
The  cleavage-cells  of  the  Morula  increase  in  number  and 
move  away  from  the  nutritive-yelk,  so  that  a  disc,  in 
the  form  of  a  watch-glass,  with  thickened  edges  (w),  is 
again  formed ;  and  a  cleavage-cavity  (s)  is  formed  between 
this  germ-membrane  (Blastoderm a,  Fig.  47,  6)  and  the 
nutritive  yelk.  After  this  the  thickened,  swollen  edge 
turns  inward,  and  a  simple  layer  of  larger,  darker-coloured 
cells  grows  from  the  edge,  centripetally  towards  the  middle 
of  the  cleavage-cavity  (Fig.  48).  The  meeting  of  these  two 
edges  at  a  central  point  gives  rise  to  the  intestinal  layer,  or 
entoderm  (Fig.  48,  i).  This  attaches  itself  immediately  to 
the  roof  of  the  cleavage-cavity,  the  cells  of  which  form  the 
skin-layer,  or  exoderm  (Fig.  49,  i).  This  completes  the 
Gastrula  of  the  Chick,  a  flatly  extended,  disc-shaped  Gas- 
trula  (Discogastruld),  resembling  that  of  the  Osseous  Fish 


GASTRULA   OF   CHICK.  221 

(Plate  III.  Fig.  24).  While,  however,  in  the  latter  case  the 
nutritive  yelk  is  attached  directly  to  the  lower  surface  of 
the  entoderm,  filling  the  whole  primitive  intestinal  cavity, 
a  low  germ-cavity  remains  between  the  entoderm  and  the 
nutritive  yelk  in  the  Disc-gastrula  of  the  Chick ;  this  is  a 
part  of  the  primitive  intestinal  cavity  (Fig.  49,  d],  and  must 
not  be  confused  with  the  cleavage-cavity  (Fig.  47,  s,  48,  s). 
The  latter  lies  between  the  nutritive  yelk  and  the  blasto- 
derm, the  former  between  the  nutritive  yelk  and  the  ento- 
derm. The  inversion  (invagination)  of  the  Gastrula  is 
complete  when  the  primitive  intestinal  cavity  has  taken 
the  place  of  the  cleavage-cavity,  the  entoderm  at  the  same 
time  attaching  its  inner  surface  to  the  inner  surface  of  the 
exoderm. 

The  germ-disc  (Blastodiscus),  which  in  an  unincubated, 
freshly-laid  Hen's  egg  lies  at  the  tread,  or  cicatricula,  is 
thus  already  a  complete  Disc-gastrula  (Discogastrula,  Fig. 
49).  It  is  plainly  visible  to  the  naked  eye,  and  appears 
like  a  small,  circular,  white  spot,  4-5  mm.  in  diameter,  in 
the  middle  of  the  upper  surface  of  the  yellow  yelk-mass. 
It  is  separated  from  the  latter  by  the  primitive  intestinal 
cavity,  and  its  thickened  edges  alone  touch  the  latter.  It 
is  possible  to  lift  up  the  entire  Gastrula.  The  two  primary 
germ-layers  are  plainly  visible  in  the  perpendicular  section  ; 
an  upper  or  outer  layer  of  smaller,  brighter  cells  forming 
the  skin-layer  (exoderm,  Fig.  49,  e) ;  and  a  lower  or  inner 
layer  of  larger,  darker  cells  forming  the  intestinal  layer 
(entoderm,  Fig.  49  z).74 

In  order  to  complete  our  survey  of  the  important  pro- 
cesses of  egg-cleavage  and  gastrulation,  we  will  now  finally 
glance  quickly  at  the  fourth  type-form  of  these  processes 


228 


THE   EVOLUTION   OF   MAN. 


superficial  cleavage  (segmentatio  superficialis,  Plate  III. 
Fig.  25-30).  This  form  is  entirely  unrepresented  among 
Vertebrates.  It,  however,  plays  the  most  important  part 


FIG.  46-49. — Gastmlation  of  a  Hen's  egg.  All  four  figures  represent 
perpendicular,  half-diagrammatic  sections  through  the  middle  of  the  thin, 
circular  tread,  or  germ-disc.  Of  the  nutritive  yelk  (n)  only  the  contiguous 
.  part  (perpendicularly  shaded)  is  represented. 

FIG.  46.—  (A)  Mulberry-germ  (Morula);  fc,  cleavage-cells. 

PIG.  47. — (B)  Germ-vesicle  (Blastula)  ;  s,  cleavage-cavity;  b,  blasto- 
derm-cells ;  w,  thickened  or  swollen  edge  of  the  germ-disc. 

FIG.  48. — (C)  Germ-vesicle  in  the  process  of  inversion  (Blastula  in- 
vaginata) ;  e,  exoderm  ;  i,  eutoderm ;  n,  nutritive  yelk ;  w,  thickened  edge  ; 
s,  cleavage-cells. 

FIG.  49. —  (D)  Gastrula  (Discogastrula)  of  Chick  :  d,  primitive  intestinal 
cavity. 


CLEAVAGE   OF   ARTICULATES.  229 

in  the  very  extensive  articulated  tribe  (Arthropoda),  in 
Insects,  Spiders,  Centipedes,  and  Crabs.  The  Gastrula 
which  results  from  this  form  of  cleavage  is  the  Bladder- 
gastrula  (Peri-gastrula,  Plate  III.  Fig.  29). 

In  eggs  which  undergo  this  superficial  cleavage,  just  as 
in  the  eggs  which  have  been  mentioned,  those  of  Birds, 
Reptiles,  Fishes,  and  other  animals,  the  formative  yelk  is 
quite  distinct  from  the  nutritive ;  and  the  former  is  alone 
concerned  in  the  cleavage,  which  does  not  touch  the  latter. 
But  while  in  those  eggs,  the  cleavage  of  which  is  discoidal, 
the  formative  yelk  is  eccentric,  and  lies  at  one  pole  of  the 
single  axis  of  the  egg,  while  the  nutritive-yelk  is  massed 
together  at  the  other  pole ;  in  those  eggs,  on  the  contrary, 
which  undergo  a  superficial  cleavage,  we  find  that  the 
formative  yelk  is  spread  over  the  whole  surface  of  the  egg, 
surrounding  the  nutritive  yelk  in  the  form  of  a  bladder, 
which  is  central,  and  situated  in  the  middle  of  the  egg. 
The  cleavage,  as  it  affects  only  the  former,  not  the  latter, 
is  naturally  entirely  superficial;  the  provision,  which  is 
massed  in  the  centre,  is  entirely  untouched  by  it.  Other- 
wise, this  superficial  cleavage  proceeds  quite  regularly,  like 
the  original  cleavage,  in  geometrical  progression.  (Plate 
III.  Fig.  25-30  represents  several  stages  of  this  process  in 
perpendicular  meridian  section  through  the  ellipsoid  egg  of 
a  Crab,  Peneus.}  The  parent-cell,  or  cytula  (Plate  111. 
Fig.  25),  first  parts  into  two  similar  cells;  from  these,  by 
repeated  simultaneous  division,  arise  first  4  (Fig.  26),  then 
8,  then  16  (Fig.  27),  64,  128,  and  so  on.  Finally,  the  whole 
formative  yelk  parts  into  numerous,  small,  similar  cells, 
which  lie  side  by  side  in  a  single  layer  over  the  whole 
surface  of  the  egg,  forming  a  superficial  germ-inembrano 


230  THE   EVOLUTION   OF   MAN. 

(Blastoderrna,  Fig.  28,  6).  This  germ-membrane  is  a  simple^ 
completely  closed  vesicle,  the  space  within,  being  wholly 
filled  with  nutritive  yelk.  The  chemical  quality  of  the 
contents  of  this  true  germ-vesicle,  or  Blastula  (Fig.  28) 
alone  distinguishes  it  from  the  Blastula  of  the  primordial 
cleavage-process  (Plate  II.  Fig.  4).  The  latter  contains 
Avater,  or  jelly  as  transparent  as  water;  the  former  con- 
tains a  dense  mixture  of  albuminous  and  fatty  substances, 
in  which  there  is  much  nutritive  matter.  As  this  extensive 
nutritive  yelk  occupies  the  centre  of  the  egg  from  the  very 
beginning  of  the  cleavage,  there  is  naturally  no  difference  in 
this  case  between  the  mulberry-germ  and  the  vesicular 
germ. 

When  the  germ-vesicle  (Fig.  28)  is  quite  complete,  the 
important  process  of  inversion  (invaginatio),  which  produces 
the  Gastrula,  follows  (Fig.  29).  A  circular,  groove-like 
deepening  first  arises  at  a  point  on  the  surface,  and  this 
enlarges  into  a  cavity,  the  primitive  intestinal  cavity  of 
the  Gastrula  (Fig.  29,  d) ;  the  point  at  which  the  inversion 
takes  place  forming  the  primitive  mouth  of  this  cavity  (o). 
The  inverted  portion  of  the  germ-membrane,  the  cells  of 
which  enlarge  and  assume  a  slender  cylindrical  form,  consti- 
tutes the  intestinal  layer  and  surrounds  the  cavity  of  the 
primitive  intestine.  The  superficial,  iminverted  portion  of 
the  germ -membrane  forms  the  skin-layer;  the  cells  of  this, 
owing  to  continual  self-division,  become  smaller  and  more 
flattened.  The  space  between  the  skin-layer  and  the  intes- 
tinal layer  (the  remnant  of  the  cleavage-cavity)  continues 
full  of  nutritive  yelk,  which  is  now  gradually  consumed. 
This  is  the  only  essential  point  in  which  the  Bladder- 
gas  trula  (Peri-gastrula,  Fig.  29)  differs  from  the  original 


RELATION   OF   THE   CLEAVAGE-FORMS.  2$! 

form,  that  of  the  Bell-gastrula  (Arcld-gastrula,  Fig.  6).  It 
is  evident  that  the  former  has  gradually  originated  from  the 
latter,  in  the  course  of  a  long  period  of  time,  by  the  accu- 
mulation of  nutritive-yelk  in  the  centre  of  the  egg.75 

The  fact  that  we  have  been  thus  enabled  to  retrace  all 
the  numerous  and  multiform  phenomena  in  the  germination 
of  different  animals  to  these  four  type-forms  of  egg-cleavage 
and  gastrulation,  may  be  regarded  as  an  advance  of  the 
widest  significance.  Of  these  four  type-forms  we  have  been 
able  to  declare  that  one  is  the  original,  palingenetic  form, 
and  that  the  other  three  are  kenogenetic  forms  descended 
from  the  first.  The  unequal,  discoidal,  and  the  superficial 
forms  of  cleavage  have  evidently  all  originated,  in  conse- 
quence of  secondary  adaptation,  from  the  primary,  original 
cleavage ;  and  we  must  consider  that  the  most  important 
cause  of  their  origin  was  the  gradual  formation  of  a  nutri- 
tive-yelk, and  the  distinction,  which  is  always  appearing  in 
an  earlier  stage,  between  the  animal  and  the  vegetative 
parts  of  the  egg,  between  the  skin-layer  and  the  intestinal 
layer.  The  inter-relation  of  the  four  cleavage-forms,  with 
regard  to  the  ordinary  distinction  between  total  and  partial 
egg-cleavage  is  as  follows : — 


I.  Palingenetic     (1.   Original     cleavage     (Bell-1 
gastrula). 

2.  Unequal    cleavage    (Hood- 


cleavage. 


A.  Total  cleavage  (with- 
out any  independent 
nutritive  yelk). 


gastrula). 
II.  Kenogenetic 

(modified  by         3.  Discoidal    cleavage    (Disc-^ 
laptation)  gastrula).  B.  Partial  cleavage  (with 


an  independent  nu- 


4.  Superficial  cleavage  (Blad- 1          tritive  yelk), 
der-gastrnla). 


The  lowest  known  intestinal  animals  (Metazoa),  that  is  to 


232  THE   EVOLUTION   OF  MAN. 

say,  the  low  Plant-animals  (Sponges,  simplest  Polyps,  etc.); 
remain  throughout  their  life  stationary  in  a  structural  stage 
which  differs  very  little  from  the  Gastrula;  their  whole  body 
being  composed  of  only  two  cell-strata  or  layers.  This  fact  is 
of  the  very  greatest  significance.  For  we  see  that  Man,  and 
indeed  all  Vertebrates,  pass  quickly  through  a  transitory 
two-layered  structural  stage,  which  is  persistently  retained 
throughout  life  by  these  lowest  Plant-animals.  By  now 
again  applying  our  first  principle  of  Biogeny,  we  im- 
mediately obtain  the  following  very  important  conclusion  : 
Man  and  all  those  other  animals,  which  in  the  first  stages  of 
their  individual  evolution  pass  through  a  two-layered  struc- 
tural stage  or  a  Gastrula-form,  must  have  descended  from  a 
primcBval,  simple  parent-form,  the  whole  body  of  which 
consisted  throughout  life,  as  now  in  the  case  of  the  lowest 
Plant-animals,  only  of  tivo  different  cell-strata  or  germ- 
layers.  To  this  most  important  primaeval  parent-form,  to 
which  we  shall  presently  refer  in  detail,  we  will  now  pro- 
visionally give  the  name  of  the  Gastrsea  (i.e.  primitive  intes- 
tinal animal).24 

According  to  the  Gastrsea  theory,  there  is  in  all  animals 
one  organ  which  is  originally  of  the  same  morphological 
and  physiological  significance ;  this  is  the  primitive  intes- 
tine ;  the  two  primary  germ-layers,  which  form  the  wall  of 
this  intestine,  must  therefore  in  all  cases  be  regarded  as  also 
of  the  same  significance,  or  as  "  homologous."  This  import- 
ant "  homology  of  the  two  primary  germ-layers  "  is,  on  the 
one  hand,  demonstrated  by  the  fact  that  the  Gastrula  in  all 
cases  originates  in  one  way,  that  is,  by  the  inversion  (in- 
vagination)  of  the  Blastula ;  and,  on  the  other  hand,  by  the 
fact  that  in  all  cases  the  same  fundamental  organs  arise 


HOMOLOGY   OF   THE   GERM-LAYERS.  233 

from  the  two  germ -layers.  The  outer  or  animal  germ -layer, 
the  skin-layer,  or  exoderm,  always  forms  the  outer  body- 
wall  with  the  most  important  organs  of  animal  life;  the 
skin-covering,  nerve-system,  organs  of  the  senses,  etc.  On 
the  other  hand,  the  inner  or  vegetative  germ-layer,  the  in- 
testinal layer,  or  entoderm,  gives  rise  to  the  inner  intestinal 
wall  with  the  most  important  organs  of  vegetative  life ;  the 
organs  of  nutrition,  of  digestion,  those  which  form  the  blood, 
etc. 

In  these  low  Plant-animals,  especially  in  Sponges, 
the  whole  body  of  which  remains  permanently  stationary 
in  the  same  structural  stage,  these  two  functional  groups 
(the  animal  and  the  vegetative  acts)  also  continue  strictly 
distributed  between  the  two  simple,  primary  germ-layers. 
Throughout  life  the  outer  or  animal  germ-layer  retains  the 
simple  significance  of  a  covering  (an  outer  skin),  and,  at 
the  same  time,  accomplishes  the  movements  and  sensations 
of  the  body.  On  the  other  hand,  the  inner  cell-stratum, 
or  the  vegetative  germ-layer,  always  retains  the  simple 
significance  of  an  intestinal  epithelium,  a  nutritive  in- 
testinal cell-stratum,  and  in  addition  to  this  appears  only 
to  produce  the  reproductive  cells.40 

In  all  other  animals,  and  especially  in  all  Vertebrates, 
the  Gastrula  appears  only  as  a  very  transitory  germ-stage. 
The  two-layered  stage  of  their  germ-rudiment  changes 
quickly,  first  into  a  three-layered,  and  then  into  a  four- 
layered  stage.  On  the  completion  of  the  germ-layers,  which 
lie  one  over  the  other,  we  have  again  provisionally  attained 
a  fixed  and  definite  point  of  view ;  and  one  from  which  we 
may  trace  and  explain  the  incidents  in  the  construction, 
which  are  much  more  obscure  and  intricate.  Trustworthy 


234  THE   EVOLUTION   OF   MAN. 

researches  by  many  observers,  embracing  the  Ontogeny 
of  the  most  diverse  higher  animals,  have  now  established 
the  important  fact  that  the  germ  in  a  certain  stage  is 
composed  of  four  secondary  germ-layers.  It  is  most  im- 
portant to  notice  that  this  is  quite  as  true  of  Man  as  of 
other  Mammals. 

In  many  cases  there  is  a  three-layered  stage  interme- 
diate between  the  two  and  the  four-layered  condition.76 
But  in  proportion  to  the  certainty  of  this  conclusion, 
that  there  are  at  first  two,  and  afterwards  four  layers,  it  is 
difficult  to  understand  the  way  in  which  these  four 
secondary  layers  arose  from  the  two  primary  layers.  In 
this  respect  the  opinions  of  the  many  observers  who  have 
studied  the  question  are  so  contradictory  that  comparison 
of  them  fails  to  enable  us  to  reach  the  truth.  There  is, 
however,  no  doubt  of  the  one  fact,  that  these  four  layers 
result  solely  from  the  two  original  germ-layers,  and  that 
they  are  not  partly  independent  of  the  latter,  as  Reichert, 
His,  and  other  confused  observers  have  asserted.71  But  the 
question  yet  remains  undecided  whether  the  two  middle 
layers  both  originate  from  one  of  the  two  primary  layers 
(from  the  outer  or  the  inner),  or  whether  one  of  the  two 
middle  layers  must  be  referred  to  the  upper,  the  other  to 
the  lower  of  the  primary  germ-layers. 

In  order  to  show  the  importance  of  this  question  to 
the  whole  history  of  evolution,  I  will  now  briefly  indi- 
cate the  significance  of  the  two  middle  layers.  We  must 
call  these  two  middle  layers  the  second  and  the  third, 
numbering  the  four  secondary  germ-layers  in  order  from 
the  outer  tc  the  inner.  The  outer  skin,  the  muscular  mass 
or  flesh  of  the  trunk,  the  muscles,  which  move  the  body 


MIDDLE  GERM-LAYERS.  235 

and  limbs,  as  well  as  the  inner  skeleton,  or  bony  frame- 
work of  the  body,  arise  from  the  second  germ -layer,  or  the 
outer  middle  layer,  which  is  called  the  skin-muscular 
layer,  or  the  skin-fibrous  layer.  The  muscles  and  vascular 
membranes,  which  first  surround  the  inner  cellular  canal 
of  the  intestine  and  its  glands,  and  which  accomplish  the 
digestive  movements  of  the  throat  (pharynx},  oesophagus, 
the  stomach,  and  the  various  other  parts  of  the  intestinal 
canal,  are  all  produced  from  the  third  germ-layer,  the 
inner  middle  layer,  which  is  called  the  intestinal-muscular 
layer,  or  the  intestinal-fibrous  layer;  the  heart  and  the 
most  important  blood-vessels  also  originate  in  this.  The 
two  middle  layers,  therefore,  especially  provide  those  cell- 
strata  which  are  employed  in  the  formation  of  the  fibrous 
coverings,  and  of  the  flesh  or  muscles.  The  cells  of  the 
second  layer  change  into  the  flesh  and  the  bony  framework 
of  the  trunk;  the  cells  of  the  third  layer  change  into  the 
muscles  and  the  fibrous  coverings  of  the  intestinal  canal. 
Both  middle  or  fibrous  layers  are  therefore  called  muscular, 
or  flesh-layers  ;  the  outer  is  called  the  skin-muscular  layer, 
because  it  lies  on  the  first  secondary  layer,  the  skin-sensory 
layer;  the  inner  is  called  the  intestinal-muscular  layer,  as 
it  lies  next  to  the  fourth  secondary  layer,  the  intestinal- 
glandular  layer  (Fig.  50). 

Baer  was  the  first  naturalist  who  recognized  and  clearly 
distinguished  the  four  secondary  germ-layers  of  the  higher 
animals.  He  did  not,  however,  fully  understand  their 
origin  and  their  wider  significance,  nor  was  he  quite 
right  in  his  explanation  of  the  details  of  their  respective 
purposes.  But  in  the  main,  their  significance  did  not 

escape  him,  and  he  even  expressed  that  view  of  the  origin 

18 


236 


THE   EVOLUTION   OF    MAN. 


of  the  two  middle  layers,  which  I,  in  opposition  to  most 
other  authors,  still  hold  to  be  correct.  That  is  to  say,  he 
derived  each  middle  layer  separately  from  a  primary  germ- 
layer  (by  fission),  and  said,  that  the  outer  or  animal  germ- 


FIG.  50.— Transverse  section  through  the  embryo  of  an  Earth-worm :  hs, 
skin-sensory  layer ;  hm,  skin-fibrous  layer ;  df,  intestinal-fibrous  layer ;  dd, 
intestinal-glandular  layer ;  a,  intestinal  cavity ;  c,  body-cavity,  or  Cosloma ; 
n,  nerve-centres  ;  it,  primitive  kidneys. 

FIG.  51. — Corresponding  section  of  the  larva  of  Amphioxns  (after 
Kowalevsky).  The  letters  indicate  the  same  parts  as  in  Fig.  50. 

layer  separates  into  two  strata,  a  skin-stratum  and  a  flesh- 
stratum ;  similarly  the  inner  or  vegetative  germ-layer 
separates  into  two  strata;  the  vascular  stratum  and  the 
mucous  stratum.  In  the  following  table  this  view  of  Baer, 
which  I  believe  to  be  right  in  regard  to  the  phylogenetic 
origin  of  the  middle  layers,  is  compared  with  the  newer 
nomenclature,  which  is  now  in  vogue  : — 

A.   The  two  primary  germ-layers.  B.  The  four  secondary  germ-layers. 


I.  The  outer  or  animal  germ- 

layer  (the  skin-layer,  or 
exoderm). 

II.  The  inner  or  vegetative 

germ-layer  (the  intesti- 
nal layer,  or  entoderm). 


1.  Skin-sensory  layer  (skin-stratum,  Baer). 

2.  Skin-fibrous  layer  (flesh-stratum,  Baer). 

3.  Intestinal-fibrous    layer  (vascular   stra 

turn,  Baer). 

4.  Intestinal-glandular  layer  (mucous  stra- 

tum, Baer). 


ORIGIN   OF   MIDDLE   GERM-LAYERS.  237 

Much  recent  research  by  Kowalevsky,  Ray-Lankester, 
Van  Beneden,  and  others  has  justified  this  "Four-layer 
Theory"  of  Baer.  For  instance,  it  can  be  plainly  shown 
that  in  the  Earth-worm  (Fig.  50),  in  the  Amphioxus  (Fig. 
ol),  and  in  some  other  animals  each  of  the  two  primary 
germ-layers  parts  into  two  secondary  germ-layers;  the 
skin,  or  outer-layer  parts  into  the  skin-sensory  layer  (hs), 
and  the  skin-fibrous  layer  (hm) :  similarly  the  intestinal 
or  inner  layer  separates  into  the  intestinal-fibrous  layer 
(df),  and  the  intestinal-glandular  layer  (dd).  The  body- 
cavity,  or  coeloma  (c),  forms  between  the  two  fibrous  layers. 

Contrary  to  this  view,  most  recent  observers  assume 
that  the  two  middle  layers  proceed  from  plane-division  of 
a  single,  middle  germ-layer  (mesoderma).  According  to 
this,  a  third  originates  between  the  two  primary  layers, 
and  by  a  secondary  process  of  fission  splits  into  two  layers 
along  the  plane  of  its  surface.  Some  observers,  however, 
as  certainly  derive  this  third  layer  from  the  lower  primary 
layer,  as  do  the  others  from  the  upper  primary  layer.  It 
is  exactly  this  suspicious  circumstance,  together  with  many 
other  grounds  (based  especially  on  Comparative  Anatomy) 
that  lead  us  to  the  conjecture,  which  I  believe  to  be  correct, 
that-  neither  party  is  right,  but  that  the  outer  middle 
layer  rather  proceeds  from  the  animal,  the  inner  middle 
layer  from  the  vegetative  germ-layer.  It  is  true,  as  we 
shall  presently  find,  that  only  a  single  middle  layer 
(Remak's  " motor-germinative  germ-layer")  usually  arises 
between  the  two  primary  germ-layers  of  mammals,  and 
that  by  the  fission  of  this,  the  two  different  middle  layers, 
the  skin-fibrous  layer  and  the  intestinal-fibrous  layer, 
originate  only  secondarily.  There  are,  however,  strong 


238  THE   EVOLUTION   OF  MAN. 

grounds  for  the  assumption  that  this  process  is  the  effect 
of  vitiated  Heredity.  The  simple  middle  germ-layer  of 
Vertebrates  has  most  probably  originated  only  secondarily 
by  the  coalescence  of  two  distinct  primary  middle  layers, 
and,  therefore,  the  fission  of  the  former  into  the  two  latter 
must  bo  regarded  as  a  tertiary  process. 

However  this  may  be,  we  have  now  reached  the  im- 
portant, definite  point  in  the  History  of  Evolution,  in 
which  the  whole  Vertebrate  body,  in  common  with  that 
of  most  higher  animals,  forms  a  tube,  the  wall  of  which  is 
composed  of  four  layers,  lying  one  over  the  other.  This 
is  not  a  figurative  comparison ;  these  constituent  parts 
of  the  tube-wall  are  actually  layers,  or  thin  plates,  which 
lie  fixed  one  over  the  other.  They  can  even  be  mechanically 
parted  or  split  off  from  each  other.  This  separability  is 
connected  with  the  fact  that  the  cells  in  each  one  of  the 
four  layers  are  alike,  while  those  of  each  are  already  in 
some  degree  distinct  or  differentiated  from  those  of  the 
other  three  layers.  The  first,  the  skin-sensory  layer,  con- 
sists of  cells  differing  from  those  of  the  second,  or  skin- 
fibrous  layer ;  the  cells  of  the  latter  are  again  different  from 
those  of  the  third,  the  intestinal-fibrous  layer;  and  these 
latter  are  of  a  somewhat  different  nature  from  the  cells  of 
the  fourth,  the  intestinal-glandular  layer.  We  find  the 
same  four  germ-layers  as  in  Man  and  other  Vertebrates 
(Fig.  51),  also  in  Soft-bodied  Animals  (Mollusca),  Articulates 
(Arthropoda),  Star-animals  (Echinoderma),  and  again  in 
the  higher  Worms  (Fig.  50).  This  fact  in  Comparative 
Ontogeny  is  of  the  greatest  phylogenetic  significance.  In 
all  cases,  these  four  secondary  germ-layers  develop  from 
the  two  primary  germ-layers ;  it  is  only  in  the  lower  Plant- 


WOLFF'S  KNOWLEDGE  OF  THE  GERM-LAYERS.      239 

animals  (Zoophytes},  especially  in  Sponges,  that  the  latter 
retain  their  original  simplicity. 

Finally,  as  a  special  proof  of  the  prophetic  genius  of 
Caspar  Friedrich  Wolff,  due  emphasis  must  be  given  to  the 
remarkable  fact  that  that  naturalist  assumed  the  existence 
of  these  four  secondary  germ-layers  under  the  name  of 
"  four  systems  formed  on  one  type,"  the  proof  of  which 
was  not  furnished  till  half  a  century  later  by  Baer.77 
(Of.  P.  46.) 


Remak's  three  germ-layei  s. 


Outer,  or 
upper 
layer 


Inner,  or 
tinder 
layer 


II.  Outer, 
germ-la 
sory  la; 


or  upper 
layer  (sen- 
sory layer) 


II.  Middle  germ- 
lay  er  ( inotor-ger- 
minative  layer) 

I  III.  Inner,  or  tinder 
germ-layer  (tro- 
phic layer) 


The  four  secondary 
germ-layers, 

1.  Skin. sensory 
layer 


2.  Skin-fibrous  layer 

3.  Intestinal-fibrous 

layer 


4. 


Intestinal -glan- 
dular layer 


The  two  primary 
germ-layers. 

Animal  layer, 
Exoderm,  or  skin 
layer. 


Vegetative  layer, 
Entoderm,  or  intes- 
tinal layer. 


Modified  ontogenctic  process. 


Original  phylogenetic  process. 


240  THE   EVOLUTION   OF  MAN. 

EXPLANATION  OF  PLATES  II.  AND  ITT. 

EGG-CLEAVAGE  AND 


These  two  plates  are  intended  to  illustrate,  by  means  of  diagrammatic 
sections,  the  most  important  differences  in  animal  egg-cleavage  and  gas- 
trulation.  Plate  II.  represents  holoblastio  eggs  (with  total  cleavage)  ; 
Plate  III.  meroblastic  eggs  (with  partial  cleavage).  The  animal  halves  of 
the  eggs  (exoderm)  are  coloured  gray ;  the  vegetative  halves  (entoderm  with 
nutritive  yelk)  red.  The  nutritive  yelk  is  perpendicularly  shaded.  All  the 
sections  are  perpendicular  meridian  sections  through  the  axis  of  the  primi- 
tive intestine.  In  all,  the  letters  indicate  the  same  parts  :  c,  parent-cell 
(Cytuld)  ;  /,  cleavage-cells  (Segmentella) ;  m,  mulberry -germ  (Morula)  ;  b, 
germ-vesicle  (Blastula)  ;  g,  oup-germ  (Gastrula) ;  s,  cleavage-cavity ;  d, 
primitive  intestinal  cavity;  o,  primitive  mouth;  n,  nutritive  yelk ;  i,  intes- 
tinal layer  (Entoderm)  ;  e,  skin-layer  (Exoderm). 

FIG.  1-6. — Original  or  primordial  egg-cleavage  of  the  lowest  Vertebrate 
(Amphioxus).  Fig.  1,  parent-cell  (Cytula)  ;  Fig.  2,  cleavage-stage  with 
4  cleavage-cells ;  Fig.  3,  mulberry-germ  (Morula)  ;  Fig.  4,  germ-vesicle 
( Blastula) ;  Fig.  5,  the  same,  in  process  of  inversion  (Invaginatio)  ;  Fig.  6, 
Hfill-gastrnla  (Archigastrula). 

FIG.  7-11. — Unequal  egg-cleavage  of  an  amphibian  (Frog).  Fig.  7, 
parent-cell  (Cytula)  ;  Fig.  8,  cleavage-stage  with  4  cleavage-cells ;  Fig.  9, 
mulberry-germ  (Morula)  ;  Fig.  10,  germ-vesicle  (Blastula) ;  Fig.  11,  Hood- 
gastrula  (Amphigastrula) . 

FIG.  12-17. — Unequal  egg-cleavage  of  a  Mammal  (Man).  Fig.  12, 
parent-cell  (Cytula) ;  Fig.  13,  cleavage-stage  with  2  cleavage-cells  (e, 
mother-cell  of  the  exoderm ;  t,  mother-cell  of  the  entoderm)  ;  Fig.  14, 
sleavage  stage  with  4  cleavage-cells ;  Fig.  15,  beginning  of  the  inver- 
sion of  the  germ-vesicle  ;  Fig.  16,  further  advanced  inversion ;  Fig.  17,  Hood- 
gastrnla  (Amphigastrula). 

FIG.  18-24. — Discoidal  egg-cleavage  of  an  Osseous  fish  (Motella?  Coitus  ?). 
The  greater  part  of  the  nutritive  yelk  (n)  is  omitted.  (Cf.  Fig.  42,  43,  pp. 
217,  219.)  Fig.  18,  parent-cell  (Cytula) ;  Fig.  19,  cleavage  stage  with 
2  cells ;  Fig.  20,  cleavage-stage  with  32  cells ;  Fig.  21,  mulberry-germ 
(Morula)  ;  Fig.  22,  germ-vesicle  (Blastula)  ;  Fig.  23,  the  same,  in  process  of 
inversion  ;  Fig.  24,  Disc-gastrula  (Discogastrula) . 

FIG.  25-30.— Superficial  egg-cleavage  of  a  Crab  (Peneus).  Fig.  25, 
parent-cell  (Cytula) ;  Fig.  26,  cleavage-stage  with  4  cells ;  Fig.  27,  cleavage- 
stage  with  32  cells ;  Fig.  28,  mulberry-germ  (Morula),  and  at  the  same 
time  the  germ-vesicle  (Blastula)  ;  Fig.  29,  Bladder-gastrula  (Perigastrula) ; 
Fig.  30,  Nauplins-germ ;  the  pharynx-cavity  has  formed  in  front  of  the 
primitive  month  (d),  owing  to  an  inversion  from  without. 


PLATE  II 


GASTRULATION. 


Amphioxus. 


Frog. 


Man. 


PLATE  /// 


GASTRULATION. 


Crab. 


TABLE    III. 


List  of  the  moat  important  differences  in  Animal  Egg-cleavage  and 
Gastrolation.79 

The  letters  a-f  indicate  the  6  animal  tribes  (the  primitive  animals  being 
excluded)  :  a,  Plant-animals  ;  b,  Worms  ;  c,  Soft-bodied  animals  (Mollusta)  ; 
d,  Star-aiiitnals  (Echinoderma)  ;  e,  Articulates  (Arthropoda)  ;  /,  Vertebrates. 


I. 

Total  Cleavage. 
Segmmtatio  totalis. 
Holoblastic  Eggs. 

Gastrula  with- 
out 
nutritive  yelk. 

UoUiyaitrula. 

I.  Original  Cleavage. 
(Segmentatio  primordialis.) 
Archiblastic  eggs. 

Bell-gastrula. 

^Archigastrula.) 
(Plate  II.  Fig.  1-6.) 

f  a.  Most   low  Plant-animals  (low 
Sponges,   Hydrapolyps,  Me- 
dusas, Corals). 
6.  Many    low    Worms    (Sagitta, 
Phoronis,      Ascidia,      many 
Nematodes,  etc.). 
c.  A  few  low  Soft-bodied  animals 
(ifollusca)—  Terebratula,  Ar- 
giope,  Pisidium. 
d.  Most    Star  -animals    (Echino- 
derma). 
e.  A  few  low  Articulates  (some 
Brancbiopods,  Copepods,  Tar- 
digrades). 
/.  The  Skull-less  Vertebrate  (Am- 
\            phioxus). 

n.  Unequal  Cleavage. 

(Segmentatio  incequalis.") 
Amphiblastic  eggs. 

Hood-gastrula. 
(Amphigattrula.') 
(Plate  II.  Fig.  7-17.) 

a.  Numerous  Plant-animals  (many 
Sponges,     Medusae,     Corals, 
Siphonopbores,  Ctenopborae). 
&.  Most  Worms, 
c.  Most  Soft-bodied  Animals  (JIol- 
lusca). 
d.  Individual   Star-animals  (vivi- 
parous  species    and   a   few 
others). 
e.  A  few  low  Articulates  (Arthro- 
poda)-both  Crustaceans  and 
Tracheates. 
/.  Cyclostoma,  Ganoids,  Amphibia, 
Mpnimals  (all  ?). 

n. 

Partial  Cleavage. 
firgmentatiopartialis. 
Meroblastic  eggs. 

m.  Discoidal  Cleavage. 
(Segmentatio  discoidalis.') 
Discoblastic  eggs. 

Disc-gastrula. 
(Disco-gastrula.) 
(Plate  III.  Fig.  18-24.) 

c.  Cuttle-fish,  or  Cephalopoda. 
e.  Some  Articulates  (Arthropoda\ 
Millepedes,    Scorpions,    and 
others. 

/.  Primitive    Fishes     (SelacJiii), 
Osseous     Fishes,     Reptiles, 
Birds  (and  Monotremes  ?). 

Gastrula  with 

nutritive  yelk. 

Mcroyastrula. 


IV.  Superficial  Cleavage.  /  o.  A  few  Sponges  (?). 
(Segmentatio  tuperjicialis  .)     1         Alcyonium  (r). 

I   6.  Individual  Worms  (?) 


Periblastic  eggs. 
Bladder-gastru'a. 


]  «.  The  great  majority  of  Artlcu- 
lates     (Arthropoda)  —  Cms- 

Myriopods'  Spidere- 


TABLE    IV. 

Systematic  Survey  of   the  five  earliest  germinal  stages  of  Animals  witt 
reference  to  the  four  different  type-forms  of  Egg-cleavage. 


A.  Total  Cleavage. 

(Segmentatio  totalis.) 

B.  Partial  Cleavage. 

(Segmentatio  partialis.) 

a.  Original  or  primor- 
dial cleavage. 

b.  Unequal  cleavage. 

c.  Discoidal  cleavage. 

d.  Superficial  cleavage. 

I.  a.  Archi- 

I.  b.  Amphi- 

I.  e.  Disco- 

I.  d.  Peri- 

monerula. 

moneruia. 

monerula. 

monerula. 

(Fig.  22  A,  p.  191.) 
A  cytod  in   which 

A     cytod     which 
Includes      formative 

A     cytod     which 
includes      formative 

A  cytod   which  in- 
cludes formative  yelk 

yelk  at   the  animal 

yelk   at   the  animal 

in    the    outer    wall, 

nutritive  yelk  are  not 
distinct. 

pole,  nutritive  yelk 
at     the     vegetative 

pole,  nutritive  yelk 
at     the      \  egetative 

nutritive  yelk  in  the 
centre. 

pole  :    the    two    are 

pole:    the    two    are 

not  very  distinct. 

quite  distinct. 

H.  «.  Archi- 

II.  b.  Amphi- 

II.  c.  Disco- 

II.  d.  Peri- 

cytula. 

cytula. 

cytula. 

cytula. 

(Plate  II.  Fig.  1.) 

(Plate  II.  Fig.  7,  12.) 

(Plate  III.  Fig.  18.) 

(Plate  III.  Fig.  25.) 

Parent-cell     which 

Parent-cell    which 

Parent-cell    which 

Parent-cell     which 

has  arisen  out  of  the 

has  arisen  out  of  the 

has  arisen  out  of  the 

has  arisen  out  of  the 

archi-monerula  by  the 
formation  of  the  pa- 

amphi-moriemla   by 
the  formation  of  the 

disco-monerula.     by 
the  formation  of  the 

peri-monerula  by  the 
formation  of  the  pa- 

rent-kernel. 

parent-kernel. 

parent-kernel. 

rent-kernel. 

III.  a.  Archi- 

III.  b.  Amphi- 

HI.  c.  Disco- 

HI.  d.  Peri- 

mornla. 

morula. 

morula. 

morula. 

(Plate  II.  Fig.  3.) 

(Plate  II.  Fig.  9.) 

(Plate  III.  Fig.  21.) 

(Plate  III.  Fig.  27.) 

A   solid  (generally 

A    roundish   heap 

A  flat  disc,  com- 

A closed  vesicle  :  a 

globular)      heap     of 

formed  of  two  kinds 

posed  of  similar  cells 

cellular  stratum  sur- 

similar cells. 

of  cells,  the  animal 

on   the  animal  pole 

rounds  the  whole  cen- 

cells at  one,  the  vege- 

of nutritive  yelk. 

tral  nutritive  yelk. 

tative   cells   at    the 

other  pole. 

IV.  o.  Archi- 

IV.  b.  Amphi- 

IV.  c.  Disco- 

rv.  d.  Peri- 

blasti'la. 

blastula. 

blastula. 

blastula. 

(Plate  II.  Fig.  4.) 

(Plate  II.  Fig.  10.) 

(Plate  III.  Fig.  22.) 

(Plate  III.  Fig.  28.) 

A   hollow   (usually 

A  roundish  vesicle, 

A  roundish  vesicle, 

A  closed  vesicle  :  a 

globular)  vesicle,  the 
wall  of  which  i  onsists 

the  wall  of  which  at 
the  animal  pole  con- 

the  small  hemisphere 
of  which  consists  of 

cell    layer   surrounds 
the    whole    nutri  ive 

of  a  single  layer  of 

si.-ts  of  smaller  cells, 

cleavage  -  cells,     the 

yelk(=Peri-morula). 

similar  cells. 

at     the     vegetative 

larger    of    nutritive 

pole  of  larger  cells. 

yelk. 

V.  a.  Archi- 

gastrula- 

V.  b.  Amphi- 

gastrula. 

V.  c.  Disco- 

gastrula. 

V.  d.  Peri- 

gastrula 

Bell-gastrula, 

Hood-gastrula. 

Disc-gas  trula. 

Bladder-gastrula. 

(Plate  11.  Fig.  6.) 
Fig.  23-28,  p.  193. 
Primitive  intestine 
empty,   without   nu- 
tritive   yelk.        Pri- 

(Plate II.  Fig.  11,  17.) 
Fig.  32-35,  p.  206, 
Fig.  41. 
Primitive  intestine 
partly    filled    with 

(Plate  III.  Fig.  24.) 
Fig.  43,  p.  219, 
Fig.  49,  p.  228. 
Primitive  intestine 
filled    with    unseg- 

(Plate  III.  Fig.  2f>.) 
Cleavage-cavity  fill- 
ed with  unsegmeuted 
nutritive  yelk.     Pri- 
mitive  intestine   dif 

mary       germ  -  layers 
»ne-layered. 

segmented   nutritive 
yelk.      Germ-layers 

mented        nutritive 
yelk.  Flat  germ-disc. 

ferent. 

often  many-layered. 

TABLE  V. 

Systematic  Survey  of  some  of  the  most  important  rhythmical  variations  in 

Egg-cleavage.80 

Only  the  first  column  (Amphioxns)  presents  the  original,  palingenetio 
cleavage-  hythm,  in  regular  geometrical  progression.  All  the  other  columns 
show  the  descended  keuogenetio  modifications. 

«  =  parent-cells.  s  =  cleavage-cells.  e  =  exoderm-cells. 

t  =  entoderm-cells. 


I. 

Ijtncelet 
(Amphi- 
oxus). 

n. 

Amphibian 
(Frog). 

ni. 
Mammal 
(Rabbit). 

rv. 
Snail 
(Trochus). 

v. 
Worm 
(Fabricia). 

VI. 

Worm 
Cyclogena), 

Ic 

Ic 

Ic 

Ic 

Ic 

le 

2s 

2s 

2s 

(le  +  10 

2s 

2s 

(i«  +  10 

2s 

(le  +  10 

4s 

4s 

4s 
(2«  +  20 

4« 

3s 

(2e  +  10 

3s 
<2«  +  10 

8s 

8s 

(4e  +  40 

8s 
(4e  +  40 

8s 
(4e  +  40 

5s 

(4e  +  10 

4s 
(3e+  It) 

12s 

(8e  +  40 

12s 

(8e  +  40 

12s 

(8e  +  40 

6s 

4e  +  2t) 

5s 

(4e+  10 

16s 

16s 

(8e  +  80 

16s 

(8e  +  80 

20s 

(16e  +  40 

10s 

(8e  +  20 

6s 

(5e  +  10 

24s 
(16e  +  80 

24s 

(16e  +  80 

24s 
(16e  +  80 

11s 

(8e  +  30 

7s 
(6e  +  10 

320 

32s 

(16e  +  160 

32s 

(16e  +.  16i) 

40s 

(32e  +  80 

19s 

(16e  +  30 

8s 

(ft  +  10 

48s 

(32e  -f  160 

48s 
(32e  +  160 

44s 

(32e+  120 

21s 
(16e  +  50 

9s 

(8e  +  10 

648 

64s 

(32e  +  320 

64s 

(32e  +  320 

76s 

(64e  +  120 

37s 

(B2e  +  5t) 

10« 
(9«  +  10 

96s 

(64e  +  320 

96s 

(64e  +  320 

84s 
(64e  +  200 

38s 
(32e  +  60 

128s 

160s 

(128e+320| 

148s 

<128e  +  20i 

70s 
(64e  +  60 

CHAPTEE   IX. 
THE   VERTEBRATE  NATURE   OF   MAN. 

Relation  of  Comparative  Anatomy  to  Classification. — The  Family-relation- 
ship of  the  Types  of  the  Anitnal  Kingdom. — Different  Significance 
and  Unequal  Value  of  the  Seven  Animal  Types. — The  Qastraea  Theory, 
and  the  Phylogenetio  Classification  of  the  Animal  Kingdom. — De- 
scent of  the  Gastrasa  from  the  Protozoa. — Descent  of  Plant-animals 
and  Worms  from  the  Gastraea. — Descent  of  the  Fonr  Higher  Classes  of 
Animals  from  Worms. — The  Vertebrate  Nature  of  Man. — Essential  and 
Unessential  Parts  of  the  Vertebral  Organism. — The  Amphioxus,  or 
Lancelot,  and  the  Ideal  Primitive  Vertebrate  in  Longitudinal  and 
Transverse  Sections.— The  Notochord.— The  Dorsal  Half  and  the  Ven- 
tral Half.— The  Spinal  Canal.— The  Fleshy  Covering  of  the  Body. — 
The  Leather-skin  (corium). — The  Outer-skin  (epidermis). —  Body- 
cavity  (caeloma).  — The  Intestinal  Tube. — The  Gill-openings.  —  The 
Lymph-vessels — The  Blood-vessels.  —  The  Primitive  Kidneys  and 
Organs  of  Eeproduction. — The  Products  of  the  Four  Secondary  Germ- 
layers. 

"  Know  thyself  !  This  is  the  source  of  all  wisdom,  said  the  great  thinkers 
of  the  past,  and  the  sentence  was  written  in  golden  letters  on  the  temple  of 
the  gods.  To  know  himself,  Linnaeus  declared  to  be  the  essential  indis- 
putable distinction  of  man  above  all  other  creatures.  I  know,  indeed,  in 
study  nothing  more  worthy  of  free  and  thoughtful  man  than  the  study  of 
himself.  For  if  we  look  for  the  purpose  of  our  existence,  we  cannot  possibly 
find  it  outside  ourselves.  We  are  here  for  our  own  sake." — KARL  EKNST 
BAEE  (1824). 

A.  DIFFICULT  task  now  lies  immediately  before  us  in  tliis 
history  of  our  individual  development ;  we  must  trace  the 
complex  human  body  with  all  its  various  parts, — organs, 


IMPORTANCE   OF   COMPARATIVE   ANATOMY.  245 

limbs,  etc.  from  the  simple  Gastrula.  The  two  primary 
germ-layers  which  form  the  entire  body  of  the  Gastrula  fall 
by  fission  into  the  four  secondary  germ-layers,  which  have 
already  been  named ;  and  of  these  four  the  whole  complex 
form  of  the  perfected  human  and  animal  body  constructs 
itself.  It  is  so  difficult  to  understand  this  process  of  con- 
struction, that  we  will  first  look  around  us  for  an  ally 
capable  of  helping  us  over  many  obstacles. 

This  powerful  ally  is  the  science  of  Comparative 
Anatomy.  Its  object  is,  by  comparison  of  the  perfected 
bodily  forms  of  the  various  groups  of  animals,  to  discover 
the  universal  structural  laws,  in  accordance  with  which  the 
animal  body  develops ;  and  at  the  same  time,  by  critically 
determining  the  degrees  of  difference  between  the  various 
classes,  and  the  larger  groups  of  animals,  to  establish  their 
relations  to  each  other  and  to  the  whole  system.  There  was 
a  time  when  this  task  was  attempted  from  a  teleological 
point  of  view,  and  in  the  actually  existing  apt  organization 
of  animals  proof  was  sought  of  a  pre-arranged  "plan  of  con- 
struction" by  the  Creator;  but,  recently,  the  establishment 
of  the  Theory  of  Descent  has  enabled  Comparative 
Anatomy  to  go  deeper,  and  its  philosophical  task  has  de- 
veloped into  the  explanation  of  the  variety  of  organic  forms 
by  Adaptation,  and  their  similarity  by  Heredity;  it  has 
also  to  seek  to  discover  the  various  degrees  of  blood- 
relationship  in  the  graduated  and  various  form-relationships, 
and  to  prove  as  nearly  as  possible  the  genealogy  of  the 
animal  kingdom.  In  this  way  Comparative  Anatomy  is 
most  closely  allied  to  the  classification  of  organic  bodies, 
which,  starting  from  the  opposite  direction,  aims  at  thtj 
same  result. 


246  THE   EVOLUTION   OF   MAN. 

In  asking  ourselves  what  place  the  most  recent  dis- 
coveries of  Comparative  Anatomy  and  the  Science  of  Classi- 
fication, among  other  organisms,  assign  to  Man,  what  light 
is  thrown  by  a  comparison  of  developed  bodily  forms  on  the 
position  of  Man  in  the  whole  animal  system,  we  receive 
a  very  simple  and  significant  answer ;  and  this  answer 
affords  conclusions  of  extreme  importance  in  explanation  of 
the  evolution  of  the  embryo,  and  as  to  the  phylogenetic 
interpretation  of  this  evolution. 

Since  the  time  of  Cuvier  and  Baer,  since  the  great 
progress  originated  by  these  two  great  zoologists  in  the  first 
decades  of  this  century,  the  whole  animal  kingdom  has 
been  universally  held  to  be  divisible  into  a  small  number  of 
main  divisions,  or  Types.  They  are  called  types,  because 
a  certain  typical  or  characteristic  structure  of  body  is 
invariably  maintained  within  each  one  of  these  main 
divisions.81  Of  late,  since  the  Development  Theory  has  been 
applied  to  this  celebrated  Doctrine  of  Types,  it  has  been 
discovered  that  all  animals  of  the  same  type  stand  in  direct 
blood-relationship  to  each  other,  and  can  be  traced  from 
a  common  parent-form.  Cuvier  and  Baer  assumed  four 
of  these  types ;  more  recent  research  has  raised  the  number 
to  seven.  These  seven  types,  or  tribes  (Phyla}**  of  the 
animal  kingdom,  are:  (1)  the  Protozog,;  (2)  the  Plant-animals 
(Zoophytes];  (3)  the  Worms  (Vermes)-,  (4)  the  Soft-bodied 
animals  (Mollusca) ;  (5)  the  Star-animals  (Echinoderma) ; 
(6)  the  Articulated-animals  (Arthropoda);  (7)  the  Vertebrata. 

I  may  at  once  introduce  the  reader  to  the  genealogi- 
cal inter-relations  of  these  seven  types  as  I  am  fully  con- 
vinced they  are  phylogenetically  constituted.  For  this 
purpose  I  will  give  as  briefly  as  possible  the  outlines  of 


THE   DOCTEINE   OF   TYPES.  247 

my  Gastrsea  Theory,24  on  which  I  base  the  monophyletic 
genealogy  of  the  animal  kingdom,  and  which  I  am  con- 
vinced must  supersede  the  Theory  of  Types  -  which  now 
prevails.  According  to  this  Gastrsea  Theory,  which  I 
enunciated  in  the  "  Monograph  on  the  Chalk  Sponges ' 
(vol.  ii.  pp.  465-467),  the  seven  types  or  tribes  of  the  animal 
kingdom  possess  an  entirely  different  significance  and  an 
entirely  unequal  value.  Only  the  four  higher  tribes 
— Vertebrates,  Arthropods,  Molluscs,  and  Echinoderms — 
are  types  in  the  sense  of  Cuvier  and  Baer,  and  even  these 
only  in  a  limited  sense,  not  as  originally  meant  by  the 
authors  of  the  theory.  On  the  other  hand,  the  lowest 
type,  that  of  the  Primitive-animals,  is  not  really  a  "  type," 
but  the  sum  of  all  the  lowest  animals ;  it  was  from  a 
branch  of  the  Primitive-animals  that  the  Gastraea  developed. 
The  two  remaining  types,  the  Plant-animals  and  the  Worms, 
stand  between  the  Primitive-animals  and  the  four  higher 
types.  They  are  more  specialized  and  typical  than  the 
Primitive-animals,  and  less  typically  organized  and  charac- 
terized than  the  four  higher  tribes. 

The  Gastrsea  Theory  is  founded  on  the  fact  that  we 
have  proved  the  two  primary  germ-layers  to  be  the  rudi- 
mentary bodily-structure  common  to  the  six  higher^"  groups 
of  animals.  But  it  is  also  proved  that  a  single  original 
organ  is  of  the  same  use,  or  homologous,  in  all  those 
animals ;  this  is  the  intestine  (protog aster),  the  primitive 
intestinal  or  stomach  cavity,  in  its  most  simple  form.  In 
the  Gastreea  itself,  and  in  the  extant  Gastreads  (Haliphy- 
scma,  Gastrophysema),  the  entire,  simple,  spherical  or  oval 
body  consists  only  of  this  simple  primitive  cavity,  open  at 
one  pole  of  the  axis  (the  primitive  intestine  and  primitive 


248  THE   EVOLUTION   OF  MAN. 

mouth),  and  of  the  two  primary  germ-layers  which  sur- 
round it  in  their  simplest  original  form  (Entoderm  and 
Exoderm).  But  in  none  of  the  Protozoa  are  there  germ- 
layers,  and  therefore  no  primitive  intestine.  The  entire 
protozoan  body  is  formed  either  of  a  very  simple  cytod,  a 
little  shapeless  mass  of  protoplasm,  as  in  the  Monera,  or  a 
very  simple  cell,  as  in  Amoebse  and  Gregarinse,  or  a  colony 
of  simple  cytods  or  cells  (as  in  most  Protozoa).  But  in  the 
last  case  the  cells  of  this  cell-community  are  either  entirely 
homogeneous,  or  but  slightly  differentiated,  and  never 
separated  into  true  germ-layers.  A  real  intestine  never 
appears  in  the  Protozoa.  The  Infusoria,  which  reach  the 
highest  degree  of  physiological  perfection  among  Protozoa, 
do  indeed  appear  to  have  an  intestine  with  a  mouth  and 
vent.  But  as  the  entire  body,  notwithstanding  the  con- 
siderable differentiation  of  its  individual  parts,  retains  only 
the  form-value  of  a  simple  cell,  we  cannot  compare  this 
physiological  food-canal  with  its  openings,  with  the  true 
many-celled  intestine,  which  in  other  animals  are  morpho- 
logically characterized  by  their  covering  of  germ-layers.83 

We  must  therefore  primarily  divide  the  whole  animal 
"kingdom  into  two  main  divisions ;  on  the  one  side  the 
Protozoa,  without  a  primitive  intestine  or  germ-layers, 
without  yelk-cleavage  or  differentiated  many-celled  tissues ; 
on  the  other  side,  the  Intestinal  animals  (Metazoa)  with 
intestines,  with  two  primary  germ-layers,  with  yelk-cleav- 
age, with  differentiated  many-celled  tissues.  The  Intestinal 
animals,  or  Metazoa,  in  which  we  include  the  six  higher 
groups  of  animals,  have  all  descended  from  the  Gastrsea, 
the  previous  existence  of  which  may  be,  even  at  this  day, 
proved  with  certainty  by  means  of  the  Gastrula.  This 


PROTOZOA  AND  METAZOA.  249 

Gastrula,  or  intestinal  larva,  which  recurs  in  a  remarkably 
similar  form  in  the  history  of  the  individual  development 
of  the  several  groups  of  animals,  is  of  the  greatest 
significance.  From  this  Gastrula  the  lowest  Vertebrate 
develops,  just  as  the  lower  forms  of  Worms,  Soft-bodiod 
Animals,  Star-animals,  Plant-anirnals,  etc.  (Of.  Plates  II., 
Ill,  and  Fig.  22-28,  pp.  191,  193.)  The  Gastrula  at  the 
present  day  presents  a  correct  picture  of  the  primitive 
Gastrsea,  which  must  have  developed  from  the  Protozoa  in 
the  Laurentian  period. 

Comparative  Anatomy  and  Ontogeny  teach  us,  further, 
that  from  this  Gastrsea  the  animal  kingdom  at  first  de- 
veloped in  two  diverging  directions  or  lines.  In  the  one 
direction  proceeded  the  low  group  of  the  Plant-animals 
(Zoophytes),  to  which  the  Sponges,  Polyps,  Corals,  Medusse, 
and  many  other  marine  animals  belong ;  and  among  fresh- 
water animals  the  well-known  Hydra,  or  fresh-water  Polyp, 
and  the  Spongilla,  or  fresh-water  Sponge.  In  the  other 
direction,  the  very  important  group  of  the  Worms,  in  the 
narrower  sense  in  which  the  present  zoological  classification 
limits  this  group,  developed  from  the  Gastrsea.  In  the 
Linnsean  system,  and  generally  in  earlier  times,  all  the 
lower  animals,  Infusoria,  Worms,  Soft-bodied  Animals, 
Plant-animals,  Star-animals,  etc.,  were  included  under  the 
name  of  Worms ;  the  name  is  now,  however,  much  more 
narrowly  restricted  to  the  true  Worms.  Under  it  are  in- 
cluded Earth-worms,  Leeches,  Ascidians,  and  also  the 
various  parasitic  Worms,  Tape-worms,  Round- worms, 
Trichinae,  etc.  Different  as  all  these  worms  appear,  in  their 
perfect  state,  they  can  all  be  traced  back  to  the  Gastnea. 
(Cf.  Table  XYIII.  in  Chap.  XVII.) 


25O  THE   EVOLUTION   OF   MAN. 

We  must  look  for  the  original  parent-form  of  the  foui 
higher  tribes  of  animals  among  the  numerous  branch-forms 
of  the  Worm  Tribe.  The  comparative  Anatomy  and 
Ontogeny  of  these  four  tribes  certainly  teach  that  all  origi- 
nated from  four  different  branches  of  Worms.  This  tribe  is 
the  common  ancestral  group  of  the  four  higher  animal  tribes. 
These  last  are :  (1)  the  Star-animals  (Echinoderma — Star- 
fishes, Sea-urchins,  Sea-lilies,  Sea-cucumbers) ;  (2)  the  im- 
portant class  of  the  Articulated-animals  (Arthropoda — 
Crabs,  Spiders,  Centipedes,  Insects) ;  (3)  the  Soft-bodied- 
animals  (Mollusca — Lamp-shells,  Mussels,  Snails,  etc.) ;  and 
finally  (4)  the  Vertebrata,  the  most  highly  developed  tribe 
of  animals,  to  which  Man  belongs. 

These  are  the  principles  of  the  unified  or  monophyletic 
genealogy  of  the  animal  kingdom,  as  they  present  them- 
selves, provisionally,  according  to  the  Gastrsea  Theory,  at 
the  present  stage  of  zoological  classification  and  of  embryo- 
logical  knowledge.  If  I  am  right  in  asserting  the  original 
similarity  or  homology  of  the  primitive  intestine  and  the 
two  primary  germ-layers  enclosing  it  in  all  intestinal 
animals,  this  phylogenetic  classification  of  the  animal 
kingdom  may  supersede  the  systems  hitherto  based  on  the 
Type  Theory.  According  to  this,  therefore,  the  seven 
types  of  that  theory  acquire  a  wholly  different  significance. 
Of  these  seven  tribes  (Phyla),  (1)  that  of  the  Protozoa 
remains  at  the  foot  of  the  scale;  from  it  springs  (2)  the 
Gastrsea,  which  branches  into  the  two  lines  of  the  Plant- 
animals  and  Worms ;  and  from  the  Worms  develop  (3)  the 
four  higher  groups  of  animals ;  these  last  are  four  diverging 
lines,  which  are  only  connected  together  at  the  base,  among 
the  lowest  Worms,  but  are  not  otherwise  comparable. 


MO.NOPHYLETIC  GENEALOGY.  25 1 

In  specially  observing  the  position  of  Man  in  the  animal 
system,  it  cannot  be  doubted  for  a  moment  that  the  entire 
bodily  structure  of  Man  is  that  of  a  Vertebrate,  and  that 
Man  possesses  in  the  characteristic  position  and  combination 
of  his  organs  all  those  peculiarities  which  appear  only  in 
the  Vertebrate  class,  and  are  totally  wanting  in  all  other 
animals.  The  Vertebrates  are  either  in  no  way  related  to 
the  three  other  higher  groups  of  animals,  or  they  are  so 
only  in  their  common  descent  from  the  Worms  and  from 
the  Gastrtea ;  on  the  contrary,  a  relationship  really  exists, 
and  may  be  clearly  proved  between  Vertebrates  and  some 
forms  of  Worms.  I  may  now  enunciate  the  proposition, 
which  we  shall  hereafter  prove,  that  the  entire  Vertebrate 
tribe  has  developed  from  the  Worm  tribe.  On  the  other 
hand,  the  Vertebrates  have  certainly  not  descended  from 
the  Articulated-animals  (Arthropoda),  the  Soft-bodied 
Animals  (Mollusca),  or  Star-animals  (Echinoderma).  There- 
fore by  far  the  greater  part  of  the  animal  kingdom  may  be 
entirely  overlooked  in  our  future  investigations,  whether 
Ontogenetic  or  Phylogenetic.  We  have  nothing  further  to 
do  with  these.  The  three  groups  which  alone  interest  us, 
are  the  Primitive  Animals  (Protozoa),  the  Worms,  and  the 
Vertebrates. 

Those  people  who  regard  the  descent  of  Man  from  the 
animal  kingdom  as  a  more  or  less  degrading  stigma,  and 
axe  ashamed  of  it,  may  take  such  consolation  as  they  can 
from  the  fact  that  the  greater  part  of  the  animal  kingdom 
is  not  akin  to  them.  The  Vertebrates  have  no  connection 
with  the  great  group  of  Articulated-animals  (Arthropoda)  ; 
but  to  the  latter  belong  not  only  the  Crabs,  but  also  the 

Spiders  and  Insects,  which  last  form  a  single  class,  eom- 
19 


252  THE   EVOLUTION   OF   MAN. 

prising  probably  as  many,  if  not  more,  distinct  species  than 
all  the  other  classes  of  animals  together.  Unfortunately, 
we  lose  by  this  the  relationship  which  might  otherwise 
connect  us  with  Termites,  Ants,  Bees,  and  other  virtuous 
members  of  the  Articulate  class.  Among  these  insects  aro 
many  well-known  patterns  of  virtue,  which  the  fable 
writers  of  old  classic  times  held  up  as  examples  for  men. 
In  the  civil  and  social  arrangements  of  the  Ants  especially, 
we  meet  with  highly  developed  institutions  which  we  may 
even  yet  regard  as  instructive  examples.  But  unfortu- 
nately these  highly  civilized  animals  are  not  related  to  us. 

Our  next  task  must  now  be,  to  enter  in  greater  detail 
into  the  vertebrate  nature  of  Man,  and  to  determine  the 
special  position  which  he  holds  in  the  system  of  Verte- 
brates. Here  it  is  necessary  to  point  out  the  most  essen- 
tial facts  in  the  particular  structure  of  the  vertebrate 
body ;  for,  otherwise,  we  shall  be  quite  unable  to  enter 
rightly  into  the  difficult  question  of  Ontogeny.  The  evolu- 
tion of  even  the  simplest  and  lowest  Vertebrate  from  the 
simple  Gastrula  is  so  complex  a  process,  and  is  so  difficult 
to  trace,  that  it  is  necessary  to  understand  the  principles 
of  the  organization  of  the  perfect  Vertebrate,  in  order  to 
comprehend  the  principles  of  its  evolution.  But  it  is  equally 
important  that  in  this  brief  anatomical  description  of  the 
vertebrate  organism,  we  should  stop  only  at  the  essential 
facts,  and  leave  all  others  untouched.  Therefore,  in  giving 
an  ideal  anatomical  sketch  of  the  main  form  of  the  Verte- 
brate and  its  inner  organization,  I  leave  out  all  secondary 
and  non-essential  circumstances,  and  confine  n^self  to 
those  most  essential. 

Many  particulars,  which  will  probably  appear  highly 


VERTEBRATE   NATURE    OF   MAN.  253 

important  and  essential  to  the  reader,  are  shown  by  the 
History  of  Evolution  and  Comparative  Anatomy  to  be  of 
secondary  and  subordinate  importance,  or  even  entirely  non- 
essential.  For  example,  from  this  point  of  view  the  head 
with  the  skull  and  the  brain  are  non-essential,  as  are  also  the 
extremities,  or  limbs.  It  is  true  that  these  parts  of  the  body 
possess  a  very  high — even  the  very  highest  physiological 
importance;  but  for  a  morphological  conception  of  the 
Vertebrate,  they  are  non-essential,  because  they  appear 
only  in  the  higher  Vertebrata,  and  are  wanting  in  the  lower. 
The  lowest  Vertebrates  possess  neither  a  clearly  marked 
head  with  a  brain  and  skull,  nor  extremities,  nor  limbs. 
The  human  embryo  also  passes  through  a  stage  in  which  it 
possesses  no  head,  no  brain,  no  skull,  in  which  the  trunk 
is  still  entirely  simple  and  undivided  into  head,  neck, 
breast,  and  abdomen,  in  which  there  is  no  trace  of  limbs, 
arms,  or  legs.  In  this  stage  of  evolution,  Man,  as  well  as 
every  other  higher  Vertebrate,  essentially  resembles  that 
simplest  Vertebrate  form,  which  is  represented  only  by -a 
.single  existing  Vertebrate,  retaining  the  form  throughout 
life.  This  single  lowest  Vertebrate,  which  deserves  the 
closest  consideration,  and,  next  to  Man,  must  undoubtedly 
be  called  the  most  interesting  of  all  Vertebrates,  is  the  well- 
known  Lancelet,  or  Amphioxus  (Plates  X.  and  XL).  As  we 
shall  afterwards  examine  this  animal  minutely  (in  Chapters 
XIII.  and  XIV.),  I  shall  say  but  little  about  it  now. 

The  Amphioxus  lives  buried  in  sea-sand ;  it  attains  a 
length  of  5-7  centimetres,  and  in  its  adult  condition  is 
shaped  exactly  like  a  long,  lanceloate  leaf.  It  ib,  therefore, 
called  the  Lancelet.  The  narrow  body  is  compressed  on 
\K)th  sides,  is  similarly  pointed  in  front  and  at  the  back. 


254  THE   EVOLUTION   OF  MAN. 

without  any  trace  of  external  appendages,  without  any 
division  of  the  body  into  head,  neck,  breast,  abdomen,  etc. 
Its  whole  form  is  so  simple,  that  its  first  discoverer  declared 
it  to  be  a  naked  Snail.  Not  until  much  later  (about  fo*-t.y 
years  ago)  was  the  remarkable  little  being  more  closely 
examined,  and  it  then  became  evident  that  it  is  a  true 
Vertebrate.  Later  investigations  have  shown  that  its  bearing 
a™  Comparative  Anatomy  and  human  Embryology  and 
"  Phylogeny  is  of  the  highest  importance.  For  the  Lancelot 
enables  us  to  solve  the  weighty  question  as  to  the  descent 
of  Vertebrates  from  Worms,  with  certain  lower  forms 
(Ascidia)  of  which  it  is  immediately  connected  in  its  de- 
velopment and  bodily  structure. 

Now,  if  we  make  several  sections  through  the  body  of 
the  Amphioxus, — first,  perpendicular  longitudinal  sections 
through  the  whole  body  from  front  to  back,  and  secondly,  a 
perpendicular  cross-section  through  it  from  right  to  left,  we 
shall  obtain  two  instructive  anatomical  pictures.  (Of.  Plates 
X.  and  XI.)  In  all  essential  points  they  correspond  to  the 
abstract  ideal,  which,  aided  by  Comparative  Anatomy  and 
Ontogeny,  we  are  able  to  conceive  as  the  primitive  type, 
as  the  picture  of  the  Primitive  Vertebrate;  of  that  long 
extinct  parent-form,  to  which  the  whole  Vertebrate  tribe 
owes  its  origin.  We  need  only  make  very  slight  and  im- 
material alterations  in  the  actual  sections  of  the  Amphioxus, 
in  order  to  obtain  such  an  ideal  anatomical  picture  or 
diagram  of  the  primitive  form  of  the  Vertebrate,  as  it  is 
represented  in  Fig.  52-56.  The  Amphioxus  differs  so 
little  from  this  primitive  form  that  it  maybe  accurately 
described  as  a  Primitive  Vertebrate.  (Cf.  Plates  X.  and  X[ 
with  Fig.  52-5G.)85 


THE   NOTOCHORD.  255 

In  the  longitudinal  section  of  the  type  of  the  Vertebrate, 
a  thin  hut  firm  rod,  of  cylindrical  form,  and  pointed  at  the 
posterior  and  anterior  ends  (Fig.  52,  x),  is  seen  in  the  middle 
of  the  body.  This  passes  through  the  whole  length  of  the 
centre  of  the  body,  and  represents  the  original  rudiments 
of  the  spine  or  vertebral  column.  This  is  the  notochord, 
the  chorda  dorsalis,  or  chorda  vertebralis,  which  is  also  called 
the  vertebral  chord  or  spinal  axis,  or,  briefly,  the  chorda. 
This  firm,  but  flexible  and  elastic  chord,  consists  of  a  cartila- 
ginous mass  of  cells,  and  forms  the  central  inner  axis  of  the 
skeleton  or  main  support  of  the  body ;  it  occurs  exclusively 
in  Vertebrates,  and  is  entirely  wanting  in  all  other  animals. 
As  the  first  rudiment  of  the  spine,  it  possesses  the  same  sig- 
nificance in  all  Vertebrates,  from  the  Amphioxus  to  Man 
But  in  the  Amphioxus  alone  the  notochord  is  retained, 
throughout  life,  in  its  simplest  form.  In  Man  and  all  the 
other  higher  Vertebrates,  on  the  contrary,  it  is  found  in  this 
form  only  in  the  earliest  embryonic  stages,  and  afterwards 
develops  into  the  articulated  vertebral  column. 

The  spinal  axis,  or  notochord,  is  the  fixed  main  axis  of 
the  Vertebrate  body,  corresponding  with  the  ideal  axis  of 
length,  and  at  the  same  time  serving  as  a  sure  guide  by 
which  we  learn  the  true  bearing  of  the  typical  relative  posi- 
tions of  the  most  important  organs  of  the  Vertebrate  body. 
By  means  of  it  we  can  picture  the  body  of  the  Vertebrate  in 
its  original  natural  arrangement,  in  which  the  axis  of  length 
lies  horizontally ;  the  dorsal  side  lies  above,  and  the  ventral 
side  below  (Fig.  52).  If  we  make  a  vertical  section  through 
the  whole  length  of  this  axis,  the  whole  body  separates  into 
two  similar  and  symmetrical  parts,  the  right  and  left  halves. 
In  both  halves  exactly  the  same  organs  originally  lie,  in  tho 


256 


THE   EVOLUTION    OF   MAN. 


FIG.  53. 


FIG.  54. 


FIG.  52. — The  ideal  Primitive  Vertebrate  type,  seen  from  the  left  side  : 
mr,  medullary  tube ;  x,  chorda ;  na,  nose ;  au,  eyes ;  g,  ear-vesicle ;  md, 
mouth  ;  k,  gill.body  ;  ks,  gill-openings ;  kg,  gill-arches ;  ma,  stomach ;  I, 
liver ;  d,  small  intestine ;  of,  anus ;  v,  intestinal  vein ;  hz,  heart ;  a,  body, 
artery;  n,  primitive  kidney  canal;  e,  ovary;  h,  testicles;  c,  body-cavity 
(visceral  cavity)  ;  ms,  muscles ;  Ih,  leather-skin  (coriuni) ;  oh,  outer-skin 
(epidermis)  ;  f,  skin-fold,  acting  as  fin. 

FIG.  53. — Same  as  above,  viewed  from  the  ventral  side. 


BILATERAL   FORM    OF   VERTEBRATES.  2$7 

PIG.  54. — Transverse  section  of  the  same  in  the  anterior  part  (through 
the  gill-body,  at  leg,  Fig.  53).  ' 

FIG.  55. — Transverse  section  of  the  same  in  the  central  part  (in  the 
neighbourhood  of  the  heart,  at  hz,  Fig.  53). 

FIG.  56. — Transverse  section  of  the  same  in  the  posterior  part  (through 
the  ovary,  at  e,  Fig.  53).  The  letters  indicate  the  same  parts  in  all  the 
sections. 

same  relative  position  and  connection ;  but  their  positions 
in  relation  to  the  central  plane  of  section  are  exactly  re- 
versed ;  the  left  half  resembles  the  right,  as  though  reflected 
in  a  mirror.  The  two  halves  are  called  counterparts,  or 
antimera.  The  perpendicular  line  of  section  which  divides 
the  two  halves,  passes  from  the  back  to  the  abdomen,  and  is 
called  the  sagittal  or  dorso-ventral  axis.  If,  on  the  other 
hand,  we  make  a  horizontal  section  lengthwise  through  the 
chord,  the  whole  body  falls  into  a  dorsal  and  a  ventral  half. 
The  line  of  section  which  passes  through  the  body  from  the 
right  to  the  left  side  is  called  the  cross  or  lateral  axis.  (Of. 
PJates  IV.  and  V.84) 

The  two  halves  of  the  Vertebrate  body  which  are 
separated  by  this  horizontal,  transverse  axis,  have  an 
entirely  different  significance.  The  dorsal  half  is  especially 
the  animal  part  of  the  body,  and  contains  the  greater  part 
of  the  so-called  animal  organs,  of  the  nerve-system,  muscle- 
system,  bone-system,  etc.  The  ventral  half,  on  the  other 
hand,  is  essentially  the  vegetative  part  of  the  body,  and 
contains  the  greater  part  of  the  vegetative  organs  of  the 
vertebrate,  the  digestive  system,  the  reproductive  system, 
etc.  The  two  outer  secondary  germ-layers  are,  therefore, 
specially  employed  in  the  formation  of  the  dorsal  half,  and 
the  two  inner  in  the  formation  of  the  ventral  half.  Each 
of  the  two  halves  develops  in  the  form  of  a  tube,  and 
surrounds  a  cavity  in  which  another  tube  is  enclosed. 


258  THE   EVOLUTION  CF   MAN. 

The  dorsal  half  encloses  the  spinal  cavity,  which  lies  above 
the  notochord,  and  contains  the  tube-shaped  central  nerve 
system,  the  spinal  marrow  or  spinal  tube.  The  ventral 
half,  on  the  other  hand,  encloses  the  much  larger  intestinal 
or  ventral  cavity,  which  lies  below  the  notochord,  and  con- 
tains the  intestinal  canal  with  all  its  appendages. 

The  spinal,  or  medullary  tube,  as  the  central  nerve 
system  or  mental  organ  of  Vertebrates  is  called  in  its  primi- 
tive condition,  consists  in  Man,  as  in  all  higher  Vertebrates, 
of  two  very  different  parts :  the  large  brain  lying  within  the 
skull,  and  the  long  spinal  cord  which  extends  from  the  brain 
along  the  whole  back  (Plate  V.  Fig.  16,  m).  But  no  part  of 
this  structure  is  seen  in  our  primitive  vertebral  type.  In  this 
the  highly  important  mental  organ,  which  occasions  the  feel- 
ing, willing,  and  thinking  of  the  Vertebrate,  appears  in  an 
extremely  simple  form.  It  is  composed  of  a  long  cylindrical 
tube  which  passes  lengthwise  through  the  body  immediately 
above  the  notochord,  and  encloses  a  narrow  central  canal  filled 
with  fluid  (Fig.  52-57,  mr).  We  find  that  the  Amphioxus 
at  the  present  day  retains  throughout  life  this  simplest 
form  of  the  spinal  canal,  just  as  it  existed  in  all  the  older 
and  lower  Vertebrates  (Plate  XI.  Fig.  15,  m).  It  is  enclosed 
in  a  tube  of  skin  which  proceeds  from,  the  immediate 
surrounding  of  the  notochord,  the  so-called  notochord 
sheath,  and  in  which,  at  a  later  period,  the  bony  vertebrae 
of  the  higher  Vertebrates  are  developed. 

Of  organs  of  sense,  the  parent-form  of  Vertebrates 
probably  possessed  an  olfactory  groove,  as  the  simplest 
rudiment  of  a  nose  (Fig.  52,  53,  no),  a  pair  of  eyes  (au\ 
and  a  pair  of  auditory  vesicles  (g)  of  the  most  simple  cha- 
racter.83 Some  of  these  organs  of  sense  are  not  represented 


THE   PRIMITIVE   VERTEBRATES. 


259 


in  the  Amphioxus,    probably  in   consequence  of  secondary 
reversion.     (Of.  Chap.  XIII.) 

Fig.  57. — Transverse  section  through  the 
anterior  part  of  the  primitive  vertebrate  type  : 
mr,  spinal  tube;  x,  chorda  (notochord)  ;  msi, 
dorsal  muscles ;  lib,  gill-vent ;  k,  gill-intestine. 

On  both  sides  of  the  spinal  tube 
of  all  Vertebrates,  and  the  notochord 
which  underlies  it,  great  niasses  of 
flesh  are  seen,  which  form  the  muscular 
parts  of  the  trunk  and  accomplish  its  movements.  Although 
in  developed  Vertebrates  these  niasses  are  differentiated  and 
combined  in  various  ways  (corresponding  to  the  variously 
differentiated  parts  of  the  bony  skeleton)  yet  in  our  ideal 
primitive  Vertebrate  we  can  distinguish  only  two  pairs  of 
main  muscles  which  traverse  the  whole  length  of  the  body 
parallel  to  the  notochord.  These  are  the  upper,  or  dorsal, 
and  the  lower,  or  ventral,  side-muscles  of  the  trunk.  The 
upper  (dorsal)  side-muscles  of  the  trunk,  the  primitive 
back-muscles  (Fig.  58,  msi)  form  the  thick  mass  of  the 
flesh  of  the  back.  The  lower  (ventral)  side-muscles,  the 
primitive  abdominal  muscles,  on  the  other  hand,  form 
the  fleshy  wall  of  the  abdomen  (Fig.  58,  mas). 


Fig.  58.  —  Transverse  section  through  the 
central  portion  of  the  ideal  Primitive  Verte- 
brate :  /,  skin-fold,  forming  fin  ;  mr,  spinal  tube  ; 
x,  chorda;  msi,  dorsal  muscles;  rasa,  ventral 
muscles  ;  a,  aorta  (in  the  mesentery)  ;  ma, 
stomach-cavity  ;  c,  body-cavity  (visceral  cavity)  ; 
hz,  heart. 


Outside    this    wall   we    find  the    outer   firm    covering 
of  the    whole    body,    called   the   leather-skin   (corium,    or 


260  THE   EVOLUTION   OF   MAN. 

cutis,  Ih).  The  lower  layers  of  this  tough  and  thick 
covering  consist  principally  of  fat  and  loose  connective 
tissue ;  the  outer  layers  of  skin-muscles  and  firmer  connec- 
tive tissues.  It  covers  the  whole  surface  of  the  fleshy  body, 
with  which  it  is  connected,  and  it  lies  immediately  below  the 
thin  outer  skin  (epidermis,  oJi).  In  the  case  of  the  higher 
Vertebrates,  hairs,  nails,  feathers,  claws,  scales  etc.,  arise 
from  this  outer  skin.  With  all  its  appendages  and  pro- 
ducts, it  consists  entirely  of  simple  cells,  and  contains  no 
blood-vessels.  Its  cells  are  connected  with  the  ends  of  the 
sensory  nerves.  Originally  the  outer  skin  (epidermis)  is  an 
entirely  simple  covering  for  the  outer  surface  of  the  body, 
and  consists  of  but  one  kind  of  cell.  In  higher  Vertebrates, 
it  afterwards  separates  into  two  strata,  an  outer,  firmer 
horn-straturn,  and  an  inner,  softer  mucous  stratum ;  many 
external  and  internal  appendages  arise  from  it  at  a  later 
period ;  the  hair,  nails,  etc.,  externally,  and  the  sweat  and 
sebaceous  glands  internally. 

In  the  primitive  Vertebrate  the  skin  probably  arose 
along  the  middle  line  of  the  body  in  the  form  of  an  erect, 
perpendicular  seam  used  for  floating  purposes  (/).  The 
Amphioxus  and  the  Cyclostomi  yet  retain  a  similar  seam, 
which  passes  almost  entirely  round  their  bodies ;  one  is  also 
found  on  the  tail  of  the  larval  Frog,  or  Tadpole  (Fig.  194). 

From  these  external  parts  of  the  vertebrate  body  we 
will  now  turn  to  the  inner  organs,  which  we  find  beneath 
the  notochord,  in  the  large  body,  or  intestinal  cavity.  To 
avoid  confusion,  we  will  in  future  call  this  cavity  the 
codom.d.  In  Anatomy  it  is  usually  called  the  pleuro-peri- 
toneal  cavity  (Fig.  58,  c).  In  Man  and  all  other  Mammals, 
but  in  no  other  animals,  this  ccelom,  when  developed,  is 


INNER   STRUCTURE   OF   PRIMITIVE   VERTEBRATES.        26l 

separated  into  two  distinct  cavities,  which  are  completely 
divided  by  a  transverse  partition,  the  muscular  midriff,  or 
diaphragm.  The  first,  or  chest-cavity,  contains  the  oesopha- 
gus, the  heart,  and  the  lungs ;  the  other,  the  ventral  cavity, 
contains  the  stomach,  small  intestine,  large  intestine,  liver, 
spleen,  kidneys,  etc.  But  in  mammalian  embryos,  these 
two  form  a  single  connected  cavity,  a  simple  ccelom,  before 
the  diaphragm  is  developed,  and  this  we  find  to  be  the 
case  in  all  lower  Vertebrates  throughout  life.  This  ccelom  is 
covered  by  a  delicate  layer  of  cells,  the  intestinal  epithelium. 
The  most  important  of  the  viscera  within  the  body- 
cavity  (coeloma),  is  the  nutritive  intestinal  tube,  the  organ 
which  forms  the  whole  body  of  the  Gastrula.  This  is  a 
long  tube,  more  or  less  differentiated,  enclosed  in  the  ccelom, 
and  having  two  openings;  a  mouth-opening  for  taking  in 
food  (Fig.  59,  60,  mcZ),  and  an  anal  opening  for  discharg- 
ing waste-matter  or  excrement  (a/).  Numerous  glrjids,  all 
of  which  proceed  from  the  intestine,  are  attached  to  the 
intestinal  canal,  which  are  of  great  importance  in  the  verte- 
brate body.  These  are  the  salivary  glands,  lungs,  liver, 
and  numerous  smaller  glands.  A  pair  of  simple  liver- 
pouches  (Fig.  59,  60,  I)  were  probably  present  even,  in  the 
parent-form  of  Vertebrates.  The  walls  of  the  intestinal 
canal  and  of  all  these  appendages,  consist  of  two  very 
different  parts  or  layers ;  the  inner  cellular  coveri  ng  is  the 
intestinal-glandular  layer,  or  the  fourth  germ-layer ;  the 
outer  fibrous  envelope,  on  the  other  hand,  proceeds  from 
the'  third  germ-layer,  the  intestinal-fibrous  layer;  it  is 
mainly  composed  of  muscle-fibres,  which  effect  the  digestive 
movements  of  the  intestine,  and  of  a  tissue  of  connective 
fibres  forming  a  firm  covering.  The  mesentery,  a  thin, 


262  THE   EVOLUTION    OF   MAN. 

ribbon-like  layer,  by  which  the  intestinal  canal  is  attached 
to  the  ventral  side  of  the  notochord,  is  a  continuation  of 
this.  In  addition  to  this,  the  most  important  parts  of  the 
blood-vessel  system,  especially  the  heart,  and  the  greater 
arteries,  also  develop  from  this  intestinal-fibrous  covering. 
In  Vertebrates  the  intestinal  canal,  as  a  whole  as  well  as 
in  its  separate  parts,  is  modified  in  various  ways,  although 
its  original  very  simple  form  is  always  the  same.  As 
a  rule,  the  intestinal  canal  is  longer,  often  many  times 
longer,  than  the  body,  and  therefore  lies,  in  many  convolu- 
tions, enclosed  in  the  cceloma,  especially  in  the  back  part. 
In  higher  Vertebrates  it  is  also  often  divided  by  valves 
into  various  separate  parts;  the  parts  being  distinguished 
as  the  mouth,  throat,  oesophagus,  stomach,  small  intestine, 
large  intestine,  and  rectum.  All  these  parts  arise  from  a  very 
simple  formation,  which  originally  (and,  in  the  Amphioxus, 
permanently)  is  a  straight,  cylindrical  canal  running  from 
front  to  rear  below  the  notochord. 

As  the  intestinal  canal,  in  a  morphological  sense,  may  be 
regarded  as  the  most  important  organ  of  the  animal  body, 
it  is  interesting  to  get  a  clear  conception  of  its  essential 
nature  in  Vertebrates,  setting  aside  all  non-essential  parts. 
In  this  respect,  it  is  especially  necessary  to  give  due 
weight  to  the  fact  that  the  intestinal  canal  in  all  Verte- 
brates shows  a  very  characteristic  division  into  two  parts, 
a  front  half  (Fig.  59,  &)  which  serves  especially  for  respira- 
tion, and  a  hind  half  which  serves  entirely  for  digestion 
(d).  In  all  Vertebrates  peculiar  clefts  appear,  at  a  very 
early  period,  on  the  right  and  left  sides  of  the  front  divi- 
sion of  the  intestinal  canal ;  these,  the  so-called  gill-open- 
ings (ks),  are  most  closely  connected  to  the  primitive 


THE   INTESTINAL   CANAL   IN    PRIMITIVE   VERTEBRATES.     263 

respiration  of  Vertebrates.  All  lower  Vertebrates,  the 
Amphioxus,  Lampreys,  and  Fishes,  continually  take  in 
water  through  the  mouth,  and  let  it  pass  out  through 


FIG.  59. — The  ideal  Primitive  Vertebrate,  seen  from  the  left  side  :  no,, 
nose  ;  au,  eye ;  g,  ear ;  md,  mouth ;  ks,  gill-openings ;  x,  chorda ;  -nir, 
spinal  tube;  kg,  gill-vessels;  fc,  gill-intestine;  hz,  heart;  ms,  muscles; 
ma,  stomach ;  v,  intestinal  vein  ;  c,  body-cavity  ;  a,  aorta  ;  I,  liver ;  d,  small 
intestine ;  e,  ovary  ;  h,  testes  ;  n,  kidney  canal ;  a/,  anus  ;  Ih,  leather  skin  ; 
oh,  outer  skin  (epidermis)  ;  /,  skin-fold,  acting  as  fin. 

the  lateral  openings  of  the  neck.  The  water  that  passes 
through  the  mouth  serves  for  breathing.  The  oxygen 
contained  in  it  is  inhaled  by  the  blood-channels  which 
extend  along  the  "  gill-arches "  (kg},  situated  between 
the  gill-openings.  These  very  characteristic  gill-openings 
and  gill-arches  are  found  in  the  human  embryo,  and  in 
the  embryos  of  all  higher  Vertebrates,  at  an  early  period 
of  their  development,  in  that  form  in  which  they  are 
retained  throughout  life  by  the  lower  Vertebrates.  In 
Mammals,  Birds,  and  Reptiles  they  never  act  as  true  organs 
of  respiration,  but  gradually  develop  into  very  different 
organs.  The  fact  that  they  originally  actually  exist  in  the 
same  form  as  in  Fishes,  is,  however,  one  of  the  most  interest- 
ing proofs  of  the  descent  of  these  three  higher  classes  of 
Vertebrates  from  the  Fishes. 


264  THE   EVOLUTION    OF   MANV 

Not  less  interesting  and  significant  is  the  circumstance 
that  the  later  respiratory  organs  of  Mammals,  Birds,  and 
Reptiles  develop  from  the  front,  or  respiratory  portion  of  the 
intestinal  canal.  A  bladder-like  fold  develops  at  an  early 
period  from  the  throat  of  the  embryo,  and  soon  takes  the 
form  of  two  large  sacs,  which  are  afterwards  filled  with 
air.  These  sacs  are  the  two  air-breathing  lungs  which  take 
the  place  of  the  water-breathing  gills.  But  this  bladder- 
like  fold,  from  which  the  lungs  arise,  is  simply  the  well- 
known  air-filled  bladder  which  is  called  the  swimming- 
bladder  in  Fishes,  and  serves  throughout  life  as  a  hydro- 
static organ,  a  swimming-apparatus  lightening  the  specific 
gravity  of  the  Fish.  Human  lungs  are  a  modification  of 
the  swimming-bladder  of  Fishes. 

The  vascular  system  of  Vertebrates  stands  in  the  closest 
morphological  and  physiological  relation  to  the  intestinal 
canal,  its  most  important  parts  being  developed  from  the 
intestinal-fibrous  layer.  It  consists  of  two  distinct  parts, 
which  are,  however,  immediately  dependent  on  each  other, 


FIG.  60. — Ideal  Primitive  Vertebrate,  ventral  view:  na,  nose;  o«, 
eyes ;  g,  ear ;  md,  month ;  fc,  gill-body ;  ks,  gill-openings ;  kg,  vascular 
gill-arches ;  hz,  heart ;  v,  intestinal  vein ;  ma,  stomach ;  I,  liver ;  d,  small 
intestine ;  of,  anus  ;  n,  primitive  kidneys ;  e,  ovary ;  h,  testicles ;  c,  body- 
cavity  ;  ms,  muscles ;  /,  skin-fold,  acting  as  float. 


THE   VASCULAR   SYSTEM   IN   PRIMITIVE   VERTEBRATES.    265 

the  system  of  blood-vessels  and  the  system  of  lymphatic 
vessels.  The  cavities  of  the  former  contain  the  red  blood ; 
those  of  the  latter,  the  colourless  lymph.  To  the  lymphatic 
system  belongs  the  coelom  (the  so-called  pleuro-peritoneal 
cavity) ;  and  also  numerous  lymphatic  ducts  which  extend 
through  a]l  the  organs,  absorbing  the  juices  which  have 
been  consumed  from  the  tissues,  and  conveying  them  into 
the  venous  blood.  Finally,  the  chyle-vessels,  which  absorb 
the  white  chyle  or  milky  nutritive  juice  prepared  by  the 
intestines,  carry  it  into  the  blood. 

The  blood-vessel  system  of  Vertebrates  is  developed  in 
various  ways,  but  seems  originally  to  have  existed,  in  the 
Primitive  Vertebrate,  in  the  simple  form  in  which  it  now 
permanently  exists  in  the  Ringed- worms  (Annelida) — for 
example,  the  common  Earth-worm — and  in  the  Amphioxus. 
Two  large  unequal  blood-channels,  which  are  originally 
situated  in  the  fibrous  wall  of  the  intestine,  and  which  run 
along  the  intestinal  canal  in  the  central  plane  of  the  body 
(one  underneath  the  intestinal  canal,  and  the  other  above), 
must  especially  be  regarded  as  essentially  and  originally  the 
most  important  part  of  the  blood-vessel  system.  These  two 
principal  channels  give  rise  to  many  branches  which  traverse 
all  parts  of  the  body,  and  pass  into  each  other  in  curves  at 
the  anterior  and  posterior  ends  of  the  body;  we  will  call 
them  the  primitive  artery  and  primitive  vein.  The  former 
represents  the  dorsal  vessels,  the  latter  the  ventral  vessels 
of  the  Worms.  The  primitive  artery  or  primordial  aorta 
(Fig.  59,  a)  lies  on  the  top  of  the  intestine,  along  the  centra"! 
line  of  the  dorsal  side,  and  conveys  oxygenated  or  arterial 
blood  from  the  gills  into  the  body.  The  primitive  or 
primordial  principal  vein  (Fig.  GO,  v)  lies  below  the  intes- 


266  THE   EVOLUTION   OF  MAN. 

tine,  along  the  central  line  on  the  side  toward  the  abdomen, 
and  conveys  carbonated,  or  venous  blood,  from  the  body 
back  to  the  gills.  In  the  front  part  of  the  gill-division  of 
the  intestine,  these  two  main  channels  are  connected  by 
several  connecting  branches,  which  rise  in  the  form  of 
arches  between  the  gill-openings.  These  "vascular  gill- 
arches  "  (£(7)  pass  along  the  gill-openings,  and  directly 
accomplish  respiration.  Immediately  behind  their  base  the 
front  end  of  the  primitive  vein  enlarges  into  a  spindle-shaped 
bladder  (hz).  This  is  the  simplest  rudiment  of  the  heart, 
which,  in  higher  Vertebrates  and  in  Man,  afterwards  as- 
sumes the  form  of  a  four-chambered,  pulsating  organ. 

In  the  lowest  part  of  the  body-cavity  of  Vertebrates, 
on  the  under  side  of  the  dorsal  wall,  near  and  on  both  sides 
of  the  notochord  and  the  mesentery,  lie  the  sexual  glands, 
which  form  the  reproductive  cells  ;  in  the  female  the  ovary, 
in  the  male  the  testis.  Recent  study  of  the  development 
of  these  parts  seems  to  show  that  the  original  formation 
of  the  sexual  glands  in  mankind  and  in  all  other  Verte- 
brates, is  hermaphroditic,  or  sexless.  The  embryonic  glands 
of  the  Vertebrate  contain  the  rudiments  of  both  kinds  of 
reproductive  organs — the  ovary  of  the  female,  which  forms 
the  ovule;  and  the  testis  of  the  male,  which  forms  the 
sperm.  These  two  kinds  of  sexual  glands,  each  of  which  at 
a  later  stage  of  development  is  distributed  to  one  only  of 
the  two  sexes,  are  originally  united  in  the  embryo.  This 
fact  leads  us  to  the  conviction,  which  appears  probable  on 
other  grounds  also,  that  Vertebrates,  in  common  with  lower 
animals,  were  originally  hermaphrodite,  that  each  indi- 
vidual was  capable  of  reproducing  itself  indepcn  lently,  and 
that  the  separation  of  the  sexual  organs  took  place  at  a 


SEXUAL   ORGANS    IN   THE   PRIMITIVE   VERTEBRATE.      26/ 

later  period.     We  may,  therefore,  assume  that  the  primitive 
Vertebrate   possessed    both    ovaries    (Fig.   60,    61,   e)   and 

testes  Qi). 

FIG.  61. — Transverse  section  through  the 
posterior  part  of  the  ideal  Primitive  Vertebrate  : 
/,  float ;  mr,  spinal  tube ;  x,  notochord ;  ms, 
muscles;  e,  ovaries;  n,  primitive  kidney  ducts; 
a.)  body-arteries  ;  d,  intestine ;  v,  intestinal  vein. 

The  sexual  organs  of  Vertebrates 
are  most  intimately  connected  with  the 
primitive  kidneys,  two  glands  running 
along  near  the  notochord,  which,  in  the  embryo,  secrete  the 
urine,  and  in  Fishes  and  Amphibia,  remain  permanently  as 
urinary  organs.87  In  higher  Vertebrates,  their  place  is  taken 
at  a  later  period  by  the  permanent  kidneys,  which  arise 
from  the  posterior  portion  of  the  primitive  kidney  ducts. 
In  their  earliest  and  simplest  form,  the  primitive  kidneys 
appear  to  be  a  pair  of  simple  ducts,  running  along  either 
side  of  the  notochord  within  the  body-cavity,  and  having 
openings  at  their  posterior  ends  (Fig.  60,  n).  In  this  form 
they  yet  appear  transiently  in  the  embryo  of  higher  Verte- 
brates, and  permanently  in  the  Worms. 

The  organs  which  we  have  thus  enumerated  in  a 
general  survey  of  the  primitive  Vertebrate,  and  have  ex- 
amined in  relation  to  their  characteristic  positions,  are 
those  parts  of  the  organism  which  are  repeated  in  all 
Vertebrates  without  exception,  in  the  same  mutual  rela- 
tions, though  they  are  modified  in  very  various  ways.  We 
have  turned  our  attention  principally  to  the  transverse 
section  of  the  body  (Fig.  54-56),  because  it  shows  most 

distinctly  the  peculiar  relative  positions  of  these  organs. 
20 


*68  THE   EVOLUTION   OF  MAN. 

But,  in  order  to  perfect  our  picture,  we  must  turn  for  a 
moment  to  pay  special  attention  to  their  articulation  or 
metameric  structure,  which  is  best  seen  in  the  longitudinal 
section  (Fig.  52,  53).  The  body  of  Man,  as  of  all  developed 
Vertebrates,  appears  to  be  composed  of  a  string  or  chain  oi 
like  members  lying  one  behind  the  other  along  the  longi- 
tudinal axis  of  the  body.  In  Man  the  number  of  these 
like  segments  or  metamera  is  about  forty ;  in  many  Ver- 
tebrates, for  example,  in  Snakes  and  Eels,  it  is  several 
hundred.  As  this  inner  articulation  corresponds  essentially 
with  the  vertebral  column  and  the  muscles  surrounding  it, 
these  members,  segments  or  metamera,  are  called  primitive 
vertebrae.  Now,  this  structure  of  these  primitive  ver- 
tebrae, or  internal  metamera,  is  correctly  regarded  as  a 
prominent  characteristic  of  Vertebrates,  and  the  various 
forms  into  which  it  is  differentiated  bear  greatly  on  the 
different  groups  of  Vertebrates.  But  in  our  present  task, 
that  of  tracing  the  development  of  the  simple  body  of  the 
primitive  Vertebrate  from  the  Gastrula,  the  segments  or 
metamera  are  of  subordinate  significance,  and  we  need  not 
deal  with  them  till  later. 

Putting  these  metamera  temporarily  aside,  I  think  that, 
in  the  above  brief  description  of  the  essential  parts,  I  have 
said  everything  necessary  as  to  the  fundamental  structure 
of  Vertebrates.  The  chief  organs  which  have  been  men- 
tioned are  the  original  and  most  important  parts,  nearly  all 
of  which  are  to  be  found,  in  a  similar  form,  in  the  adult 
Amphioxus,  and  which  re-occur  in  the  original  rudimentary 
germ  of  all  members  of  this  tribe.  Many  very  important 
parts,  which  appear  to  be  entirely  essential,  will,  it  is  true, 
be  missed  in  this  review.  As  I  have  already  remarked,  the 


METAMERA.  369 

ripecialized  head  of  the  Vertebrate  with  skull  and  brain  is 
&  non-essential,  secondary  formation  ;  and  the  same  may  be 
said  of  the  limbs  or  extremities.  Important  as  these  parts 
of  Man  and  the  higher  vertebrates  are  physiologically,  they 
are  morphologically  unimportant,  for  originally  they  were 
absent,  and  they  develop  only  at  a  later  period.  The  older 
Vertebrates  of  the  Silurian  Period  had  neither  skull  nor 
brain,  and  were  entirely  without  limbs. 

If  we  pay  no  attention  to  those  parts  which  are  second 
arily  formed,  and  are  therefore  unimportant,  and  if  we 
provisionally  examine  only  the  essential,  primary  parts,  we 
shall  greatly  simplify  our  task.  This  task  is  essentially 
to  trace  the  described  organism  of  the  "primitive  Verte- 
brate" from  the  simple  germ-form  of  the  Gastrula.  That 
simplest  Vertebrate  body  is,  as  is  usually  said,  composed  of 
two  symmetrical,  double  tubes ;  of  a  lower  tube,  the  body- 
wall,  which  surrounds  the  intestinal  tube,  and  of  an  upper 
tube,  spinal  canal,  which  surrounds  the  spinal  marrow. 
Between  the  spinal  tube  and  the  intestinal  tube,  lies  the 
notochord,  the  most  essential  part  of  the  inner  axis  of 
the  skeleton  which  characterizes  the  Vertebrate.  This 
characteristic  arrangement  of  the  most  important  organs 
re-occurs  in  all  Vertebrates  from  the  Amphioxus  to  Man, 
(Of.  Plate  IV.,  with  explanation.)  We  must,  therefore, 
now  examine  the  way  in  which  these  organs  develop  from 
the  two  primary  germ-layers  of  the  Gastrula,  and  from  the 
four  secondary  germ-layers  which  arise  by  fission  of  the  two 
primaries. 

In  order  to  solve  this  difficult  problem  it  seems  desirable 
to  begin  with,  a  statement  of  the  most  important  conclusions 
of  ontogenetic  study.  The  distant  goal  will  be  more  easily 


2/O  THE   EVOLUTION  OF   MAN. 

reached  if  we  see  it  clearly  before  us.  I  will  now,  there- 
fore, mention  as  briefly  as  possible  the  relations  which 
these  particular  organs  of  the  vertebrate  organism  bear  to 
the  four  different  germ-layers. 

The  first  of  the  secondary  germ-layers,  the  skin-sensory- 
layer,  produces, — firstly,  the  outer  covering  of  the  whole 
body ;  the  outer  skin,  or  epidermis,  and,  in  higher  Ver- 
tebrates, the  hair,  nails,  sweat  and  sebaceous  glands,  and 
all  other  parts  developing  secondarily  from  the  originally 
simple  outer  skin  (epidermis).  In  the  second  place,  from 
this  layer  arises  also  the  central  nerve-system,  the  medullary 
or  spinal  canal.  It  is  remarkable  that  this  mental  organ 
develops  from  the  outer  surface  of  the  epidermis,  and,  only 
afterwards,  during  the  course  of  the  development  of  the 
individual,  gradually  moves  inward,  so  that,  at  a  later 
period,  it  is  situated  internally,  surrounded  by  muscles, 
bones,  and  other  parts.  Thirdly,  the  primitive  kidney  of 
the  Vertebrate  which  secretes  the  urine,  probably  develops 
from  the  outer  germ-layer.  It  may  be  presumed  that  this 
primitive  kidney  was  originally  a  secretory  gland  of  the 
skin,  like  the  sweat-glands,  and,  like  them,  developed  from 
the  outer  skin  (epidermis) ;  at  a  later  period  it  lies  deep 
within  the  body. 

From  the  second  of  the  secondary  germ-layers,  the  skin- 
fibrous  layer,  arises  the  principal  mass  of  the  vertebrate 
body,  namely,  all  those  parts  lying  between  the  epidermis 
and  the  inner  coelom,  and  forming  the  firm  body-wall.  To 
these  belong,  firstly,  the  leather-skin  (corium),  which  lies 
at  the  surface  directly  under  the  epidermis, — the  firm, 
fibrous  covering  which  contains  -the  nerves  and  blood-vessels 
of  the  skin;  secondly,  the  great  masses  of  muscle  of  the 


RELATION   OF  THE   ORGANS  TO   THE   GERM-LAYERS.      2/1 

whole  trunk,  or  the  flesh,  surrounding  the  vertebral  column, 
and  consisting  of  two  main  groups  of  muscles;  the  dorsal, 
or  upper  side-muscles  of  the  trunk,  and  the  ventral,  or  lower 
side-muscles  of  the  trunk.  To  these  must  be  added,  in  the 
third  place,  the  inner  skeleton,  which  is  especially  character- 
istic of  Vertebrates,  the  central  formation  of  which  is  the 
spinal  axis  or  notochord,  developing  at  a  later  period 
into  the  articulated  vertebral  column;  also  all  the  bones, 
cartilages,  ligaments,  etc.,  which  form  the  vertebral  skeleton 
in  all  more  highly  developed  Vertebrates,  and  are  connected 
by  the  sinews  and  muscles  belonging  to  it.  Fourthly  and 
finally,  from  the  innermost  layer  of  cells  of  this  secondary 
germ-layer  develops  the  exocodar,  that  is,  the  outer,  or 
parietal  coelom-epithelium,  the  cell-layer  which  forms  the 
inner  covering  of  the  body -wall,  and  which  is  also  probably 
the  original  site  of  the  male  sexual  cells. 

The  third  secondary  germ-layer  is  the  intestinal-fibrous 
layer.  From  this  is  developed,  firstly,  the  endoccelar,  that 
is,  the  inner,  or  visceral  ccelom-epithelium,  the  layer  of 
cells,  covering  the  outer  surface  of  the  whole  intestine,  pro- 
bably also  the  site  of  the  female  sexual  cells.  Secondly,  from 
this  layer  originates  the  heart,  and  *the  great  blood-vessels 
of  the  body,  as  well  as  the  blood  itself,  so  that  it  has  been 
called,  in  a  peculiar  sense,  the  vascular  layer.  The  great 
blood-channels,  or  arteries,  going  from  the  heart  and  the 
great  veins  passing  to  the  heart,  as  well  as  the  chyle-ves- 
sels, which  open  into  the  latter,  are  formed,  like  the  heart, 
the  lymph,  and  the  blood  itself,  from  this  third  germ- 
layer.  Thirdly,  arises  the  muscular  tube  of  the  intes- 
tines, or  the  mesenteric  tube,  that  is,  the  whole  of  those 
fibrous  and  fleshy  parts  which  form  the  outer  wall  of  the 


272  THE   EVOLUTION   OF  MAN. 

intestinal  canal,  as  well  as  the  mesentery,  the  thin,  fibrous 
membrane  by  which  the  intestinal  canal  is  connected  with 
the  ventral  side  of  the  vertebral  column. 

The  history  of  the  fourth  secondary  germ-layer,  or  the 
intestinal-glandular  layer,  is  very  simple  and  clear.  Its 
only  product  is  the  intestinal  cellular  covering,  or  the  Epi- 
thelium of  the  intestinal  canal  with  all  its  appendages,  the 
large  and  small  intestinal  glands,  among  which  are  the 
lungs,  liver,  salivary  glands.  (Cf.  Plates  IV.,  Y.) 


TABLE    VI. 

Systematic  Survey  of  the  principal  organs  of  the  ideal  Primitive  Vertebrate, 
the  hypothetical  parent-form  of  Vertebrates,  and  of  their  development 
from  the  germ -lay  era. 


Primary  Germ- 
layers. 

Secondary  Germ- 
layers. 

Most  important  Organs  of 
ths  Primitive  Vertebrato. 

' 

1.  Outer    skin   (Epidermi»\    A 

L 

simple  cell-covering  of  the 

Skin-senpory  layer 

outer  surface. 
a.  Spinal    tube   (Tubut    medul- 

(Skin-stratum,  Baer), 

laris)   (with   the  organs  of 

or 

sense  :  the  nose,  eyes,  organs 

L 
Skin-layer 

Senrory  layer. 
Lamina  neurodermalit,  B. 

of  hearing). 
3.  Primitive    Kidneys    (Protone- 
phra)  fa  pair  of  simple  ducts, 

(Animal  germ-layer,    / 

primitive  kidney  ducts). 

Baer). 

lamina  dermalis,  H. 

4.  True  skin  (Corium)  (Outis  and 

Excderma. 

n. 

subcutis\ 

Skin  -fibrous  la^er 
(Flesh-stratum,  Baer), 

6.  Muscles  of  the  trnnk  (dorsal 
and  ventral  muscles). 
6.  Nolochord  (Chorda  dorsalis). 

Flesh  -layer. 

1.  Exocoelar,  or  Parietal  Co?lom- 
epitbelium  (the   inner    rpll- 

Lamina  inodei  malts,  B. 

covering  of  the  body-wall). 

\ 

8.  Male  sexual  glands  (Tferfes). 

Coeloma,  or  Body-cavity.  A  space  between  the  body-wall  and  the 
intestinal  wall,  between  the  exoderm  and  the  entoderm,  filled  with  lymph 
(colourless  blood). 


9.  Female  sexual  glands  (Ovary"). 

10.  Kndocoelar,    or   Visceral    Co3- 

lom-epithelium  (the  outer 
cell-covering  of  the  intestinal 
tube.) 

11.  Principal  blood-vessels  (primi- 

tive artery  or  dorsal  vessel, 
and  the  primitive  vein  or 
ventral  vessel). 

12.  Mesentery.       ' 

13.  Muscular    intesMne    wall  (fi- 

brous intestinal  wall). 

(u.  Intestinal    epithelium    (ttinel 
Intestinal-glandular  layer  cell-covering  of  the  intestinar 

(Mucous  stratum,  Baer),  tube). 

or  1  15.  Intestinal    glandular    epithe- 

Mucous  layer.  lium  (liver-cells  and   other 

Lam  no,  mycogastralit  H         \         intestinal  glandular  cello). 


in. 

Intestinal-fibrous  layer 

(Vascular  stratum,  Baer), 

n. 

Intestinal  laver 
(.Vegetative  germ- 

or 
Vascular  laver. 
lamina  inogastralit,  B. 

layer,  Baer).          ( 

lamina  gistralis,  B. 

Entodeima. 

TV.                          / 

CHAPTER  X. 

THE   CONSTRUCTION    OF    THE   BODY   FROM   THE   GERM- 
LAYERS. 

I'he  Original  (Palingenetic)  Development  of  the  Vertebrate  Body  frouv 
the  Gastrula. — Relation  of  this  Process  to  the  Later  (Kenogenetic) 
Germination,  as  it  occurs  in  Mammals. — The  most  important  act  in  the 
Formation  of  the  Vertebrate. — The  Primary  Germ-layers,  and  also  the 
Secondary  Germ-layers,  which  arise  by  Fission  of  the  Primaries, 
originally  form  Closed  Tubes. — Contemporaneously  with  the  Completion 
of  the  Yelk-sac,  the  Germ-layers  flatten,  and  only  later  again  assume 
a  Tubular  Form. — Origin  of  the  Disc-shaped  Mammalian  Germ-area. 
— L'ght  Germ-area  (area  pellucida)  and  Dark  Germ-area  (area 
opaca). — The  Oval  Germ-shield,  which  afterwards  assumes  the  Shape 
of  the  Sole  of  a  Shoe,  appears  in  the  Centre  of  the  Light  Germ-area 
(a.  pellucida). — The  Primitive  Streak  separates  the  Germ-shield  into 
a  Pught  and  Left  Half.— Below  the  Dorsal  Furrow  the  Central  Germ- 
layer  parts  into  the  Notochord  and  the  Two  Side-layers. — The  Side- 
layers  split  horizontally  into  Two  Layers :  the  Skin-fibrous  layer  and 
the  Intestinal-fibrous  layer. — The  Primary  Vertebral  Cords  separate  from 
the  Side-layers. — The  Skin-sensory  Layer  separates  into  Three  Parts : 
the  Horny  Layer,  Spinal  Canal,  and  Primitive  Kidney. — Formation  ol 
the  Coelom  and  the  First  Arteries. — The  Intestinal  Canal  proceeds  from 
the  Intestinal  Furrow. — The  Embryo  separates  from  the  Germ-vesicle. 
— Around  it  is  formed  the  Amnion-fold,  which  coalesces  over  the  back 
of  the  Embryo,  so  as  to  form  a  Closed  Sac. — The  Amnion. — The 
Amnion-water. — The  Yelk-sac,  or  Navel-vesicle. — The  Closing  of  the 
Intestinal  and  Ventral  Walls  occasions  the  Formation  of  the  Navel. — 
The  Dorsal  and  Ventral  Walls. 

"  The  development  of  the  Vertebrate  proceeds  from  an  axis  upward,  in 
two  layers,  which  coalesce  at  the  edges,  and  also  downward,  in  two  layers, 


GASTRULA   OF   THE   MAMMAL.  2/5 

which  likewise  coalesce  at  the  edges.  Thus  two  main  tubes  are  formed,  one 
above  the  other.  During  the  formation  of  these,  the  embryo  separates  into 
strata,  so  that  the  two  main  tubes  are  composed  of  subordinate  tubes  which 
enclose  each  other  as  fundamental  organs,  and  are  capable  of  developing 
into  all  the  organs." — KAKL  ERNST  BAEE  (1828). 

THE  mammalian  egg,  in  the  stage  of  development  in  which 
we  left  it,  presented  an  extremely  important  and  remark- 
able germ-form,  the  Gastrala  (Fig.  41,  p.  213,  and  Plate  II. 
Fig  17).  The  whole  body  of  this  globular  Gastrula  con- 
sists solely  of  the  two  kinds  of  cells  which  compose  the 
two  primary  germ-layers.  A  single  stratum  of  lighter- - 
coloured  and  firmer  cells  forms  the  outer  germ-layer,  and  con- 
stitutes an  outer  covering  over  the  whole  surface  of  the  body 
of  the  Gastrula.  The  whole  interior  of  the  latter  is  filled 
by  the  darker  and  softer  cells  of  the  inner  germ-layer :  it 
is  only  at  a  single  point  that  these  latter  cells  appear  at 
the  outer  surface  of  the  spherical  body;  this  point  is  the 
mouth  of  the  Gastrula,  the  primitive  mouth  (protostoma, 
Fig.  41,  o). 

It  is  no  easy  task  to  explain  how  the  complex  mamma- 
lian organism  originates  from  this  simple  Gastrula.  In 
order  to  lighten  the  task,  we  have,  as  a  preliminary,  made 
ourselves  acquainted  with  the  typical  structure  of  the 
simple  primitive  Vertebrate  (Fig.  52-56,  p.  256).  We  chiefly 
based  our  study  of  that  directly  on  the  real  conditions 
which  may  yet  be  actually  seen  in  the  structure  of  the 
body  of  the  lowest  extant  Vertebrate,  the  Amphioxus.  In 
most  important  points  of  internal  organization  we  may 
regard  the  Amphioxus  as  a  correct,  palingenetic  picture  of 
the  long-extinct  parent-form  of  all  Vertebrates,  the  form  to 
which  the  origin  of  Man  must  also  be  referred.  It  is  only 
in  a  few  unimportant  points  that  the  Amphioxus  appears  to 


THE   EVOLUTION   OF  MAN. 


Fro.  62-69. — Diagrammatic  transverse    sections  through  the  most  im- 
portant germ-forms  of  the  ideal  Primitive  Vertebrate  (Fig.  52-61).89 

FIG.  62. — A.  Transverse  section  through  the  Gastrula;  two-layered  germ 

FIG.  63. — B.  Three-layered  germ. 

FIG.  64. — C.  Four-layered  germ  (four  secondary  germ-layers). 


BELL-GASTRULA   OF  AMPHIOXUS.  277 

FIG.  65. — D.  The  body-cavity  appears  between  the  skin-layer  and  the 
intestinal  layer. 

FIG.  66. — E.  The  notochord  appears  between  the  spinal  furrow  and  the 
intestine. 

FIG.  67. — F.  The  primitive  kidneys  and  primitive  vertebrae  appear ;  the 
spinal  tube  is  closed. 

FIG.  68. — 0.  The  rudiments  of  the  sexual  organs  appear  near  the  primi- 
tive kidneys.  The  primitive  vertebrae  surround  the  notochord  aud  the 
spinal  tube. 

FIG.  69. — JET.  The  main  blood-vessels  appear  above  and  below  the  intestine. 

The  letters  indicate  the  same  parts  in  all :  d,  the  intestinal  cavity ;  dd, 
the  intestinal-glandular  layer;  df,  the  intestinal-fibrous  layer;  ^mesen- 
tery; y,  female  germ-glands  (rudimentary  ovary)  ;  x,  male  germ-glands 
(rudimentary  testes) ;  a,  aorta  (primitive  artery) ;  vd,  intestinal  vein 
(primitive  vein) ;  vc,  cardinal  vein ;  ch,  notochord ;  uw,  primitive  ver- 
tebrae ;  wy  vertebrae ;  rm,  dorsal  muscles ;  bm,  ventral  muscles ;  u,  primi- 
tive kidneys ;  mf,  spinal  furrow ;  mr,  spinal  tube ;  hs,  horn-plate.  In  all, 
the  four  secondary  germ-layers  are  indicated  by  shading :  the  intestinal 
glandular  layer  (dd)  is  dotted.  The  intestinal-fibrous  layer  (df)  is  per- 
pendicularly shaded.  The  skin-fibrous  layer  (hf)  is  horizontally  shaded. 
The  skin-sensory  layer  (hs)  is  b.ack. 


be  kenogenefcically  altered,  and  we  must  suppose  that 
the  conditions  were  originally  different.  This  is  equally 
true  of  the  very  important  germ-history  of  this  lowest  Ver- 
tebrate. In  a  later  chapter  (XIV.)  we  shall  enter  into  the 
details  of  this.  Here,  however,  we  may  base  our  argument 
on  this  germ-history  so  far  as  we  are  able,  from  a  compara- 
tive study  of  the  germination  of  the  various  Vertebrates,  to 
form  an  approximate  conception  of  the  course  of  individual 
evolution,  as  it  originally  occurred  in  the  oldest  and  simplest 
Vertebrates.  Only  after  we  have  gained  a  general  view  of 
this,  can  we  turn  to  the  far  harder  task  of  tracing  the 
construction  of  the  mammalian  organism,  and  especially 
that  of  Man,  from  the  Gastrula. 

The  palingenetic  Bell-gastrula  of  the  Amphioxus  (Fig 
28,  p.  193)  affords  a  safe  starting-point.     A  series  of  dia> 


278  THE   EVOLUTION   OF  MAN. 

grammatic  transverse  sections  through  those  germ-forms 
which  first  develop  from  the  Gastrula,  will  best  and  most 
easily  afford  us  the  desired  view.  (Cf.  Fig.  62-69,  and 
Plates  IV.,  V.)  In  the  first  place,  a  third  layer,  the  middle 
layer,  or  fibrous  layer  (mesoderma,  Fig.  63  mb),  arises  be- 
tween the  two  primary  germ-layers  of  the  Gastrula  (Fig. 
62).  Then,  this  three-layered  stage  is  followed  by  one  in 
which  there  are  four  layers  (Fig.  64).  As  we  have  already 
stated,  each  of  the  two  primary  germ-layers  probably 
originally  contributed  to  the  formation  of  the  middle  layer 
(mesoderma),  although  it  is  usually  asserted  that  the  latter 
originates  from  one  only  of  the  former.  It  is  probable  that 
the  exoderm,  or  skin-layer  (e),  separated  into  the  skin- 
sensory  layer  (ha)  and  the  skin-fibrous  layer  (hf),  and 
correspondingly,  the  entoderm,  or  intestinal  layer,  into  the 
intestinal-fibrous  layer  (df)  and  the  intestinal-glandular 
layer  (dd). 

When  the  four  germ-layers  are  completed,  the  form  of 
the  Gastrula,  which  had  but  one  axis,  has  become  symme- 
trically bilateral  (cf.  p.  257).  In  consequence  of  the  body 
becoming  flat,  a  distinction  is  formed  between  the  dorsal 
and  ventral  sides,  between  the  right  and  the  left.  Parallel 
with  the  axis  of  length,  a  delicate  streak,  the  indication  of 
a  furrow,  appears  in  the  centre  of  the  dorsal  side.  The  side 
walls  of  this  furrow,  which  is  called  the  "  spinal  furrow  " 
(m/),  rise  in  the  form  of  two  parallel  ledges  (Fig.  65  m/) ; 
these  are  the  spinal  swellings  (medullary  or  dorsal  swell- 
ings). Their  two  parallel  edges  bend  toward  each  other 
(Fig.  66  mf)  and  finally  coalesce,  so  that  the  trench 
becomes  a  tube ;  this  is  the  spinal  tube  (Fig.  67  rar). 
Along  the  longitudinal  axis  of  the  body,  a  solid  cylindrical 


DEVELOPMENT   OF   MAMMALIAN   GASTRULA.  2/9 

cord  arises  between  the  spinal  tube  and  the  intestinal  tube  ; 
this  is  the  notochord.  or  chorda  (cJi).  It  originates  from 
the  central  portion  of  the  skin-fibrous  layer,  while  the  side 
portions  of  the  latter  supply  the  true  skin  and  the  great 
part  of  the  flesh.  This  flesh-mass  separates  into  the  dorsal 
muscles  (Fig.  68,  69  rm)  and  the  ventral  muscles  (bin). 

The  separation  of  the  four  secondary  germ-layers  is 
followed  by  a  separation  between  the  skin-fibrous  layer  (hf) 
and  the  intestinal-fibrous  layer  (df).  Between  the  two, 
a  chink-like  cavity,  filled  with  fluid,  arises ;  this  is  the  true 
body-cavity  (cosloma,  Fig.  65-69  c).  The  intestinal  tube  lies 
freely  in  this,  being  only  supported  along  the  length  of  the 
notochord  by  a  band  of  the  intestinal-fibrous  layer,  which 
afterwards  extends  into  the  mesentery  (Fig.  68  g).  Two 
narrow  canals,  filled  with  blood,  form  within  the  intestinal- 
fibrous  layer,  and  traverse  the  whole  length  of  the  intestine, 
one  passing  underneath,  and  the  other  above;  these  are  the 
first  blood-vessels.  The  upper  of  the  two  is  the  dorsal 
vessel  (Fig.  69  a),  the  latter  is  the  ventral  vessel  (vd) ;  the 
one  afterwards  gives  rise  to  the  aorta,  the  other  to  the 
intestinal  vein  and  the  heart. 

Finally,  the  first  rudiments  of  the  urinary  and  sexual 
glands  miike  their  appearance  on  either  side  of  the  in- 
testinal tube  and  the  notochord  attached  to  the  dorsal 
wall  of  the  body-cavity.  The  primitive  kidneys  (M.)  re- 
semble two  narrow  canals,  traversing  the  body,  parallel 
to  the  notochord,  opening  at  the  anterior  end  into  the 
body-cavity,  and  at  the  posterior  end  through  the  outer 
skin  (or  in  the  last  chamber  of  the  intestine).  They 
probably  originally  arose  as  skin-glands,  formed  by  an 
inversion  of  the  skin-sensory  layer  (Fig.  66-68  u).  In 


28O  THE   EVOLUTION   OF   MAN. 

their  immediate  neighbourhood  are  the  sexual  organs,  in  the 
form  of  simple  heaps  of  cells,  which  are  attached  to  the 
body- wall,  near  the  mesentery.  Presumably,  they  originated 
as  hermaphrodite  glands,  the  female  gland  (y)  from  the 
inner,  the  male  gland  (x)  from  the  outer  germ-layer.  The 
former  becomes  the  ovary,  the  latter  the  testes.  Simul- 
taneously with  these  changes,  the  spinal  tube  has  completely 
detached  itself  from  its  original  site,  the  skin-sensory  layer, 
and  has  made  its  way  far  into  the  body.  A  sheath  has 
formed  round  the  notochord,  and  processes  from  this  "noto- 
chord  sheath  "  grow  round  the  spinal  tube,  imbedding  it  in 
a  vertebral  canal  (Fig.  68,  69  w). 

If,  for  a  moment,  we  leave  the  transverse  sections,  and 
trace  the  evolution  of  the  primitive  Vertebrate  in  longi- 
tudinal sections,  we  see  that  at  a  very  early  period  the 
intestinal  tube  is  divided  into  a  gill-intestine  and  a  stomach- 
intestine,  in  consequence  of  the  appearance  of  gills  in  the 
anterior  portion.  The  primitive  mouth  of  the  Gastrula 
closes ;  the  two  permanent  openings  of  the  future  intestine 
arise  as  new  formations  from  the  exterior ;  the  mouth  in 
front,  the  anus  behind.  Moreover,  the  outer  body-wall 
becomes  articulated,  owing  to  the  fact  that  the  fleshy  mass 
of  the  trunk-muscles  assumes  the  form  of  a  number  of 
similar,  consecutive  portions,  segments,  or  metamera.  In 
correspondence  with  these,  each  of  the  respective  portions 
of  the  nerve  and  vascular  systems  becomes  distinct. 

The  following  processes  must,  therefore,  be  emphasized 
as  the  chief  acts  by  which  the  simple  Gastrula  changes  into 
the  typical  vertebrate  organism  :  1.  The  two  primary  germ- 
layers  part  by  fission  into  four  secondary  germ-layers. 
2.  The  Gastrula  becomes  flattened,  so  that,  instead  of  a  form 


THE  MOST  IMPORTANT  PROCESSES  IN  GASTRULATION.  281 

with  a  single  axis,  it  assumes  the  bilateral  vertebrate  form. 
•\.  The  body-cavity  (cosloma)  arises,  in  consequence  of  the 
disconnection  of  the  skin-fibrous  layer  and  the  intestinal- 
fibrous  layer.  4.  Along  the  central  line  of  the  dorsal 
mi  i  face  the  nerve-centre  appears  in  the  form  of  a  trench- 
shaped  furrow;  it  then  changes  into  the  spinal-tube  and 
completely  detaches  itself  from  the  skin-sensory  layer. 
5.  Immediately  below  the  spinal  tube,  the  notochord  origi- 
nates from  the  central  part  of  the  skin-fibrous  layer,  while 
the  side  parts  of  the  same  layer  form  the  true  skin  and  the 
trunk-muscles ;  the  latter  articulate  themselves  into  meta- 
mera.  6.  In  the  outer  stratum  of  the  intestinal  wall,  in 
the  intestinal-fibrous  layer,  originate  the  main  blood-vessels, 
a  dorsal  vessel  (aorta)  above  the  intestinal  tube,  and  a  ven- 
tral vessel  (primitive  vein)  below  the  latter.  7.  The  intes- 
tinal tube  separates  into  two  main  parts ;  the  gill-intestine 
in  front,  the  stomach-intestine  behind.  Several  gill-open- 
ings form  on  either  side  of  the  gill-intestine.  8.  The 
intestinal  tube  acquires  two  new  openings,  a  mouth  in  front, 
an  anus  behind  ;  the  original  primitive  mouth  of  the  Gas- 
trula  closes.  9.  Close  by  the  intestine  and  notochord,  and 
on  either  side  of  them,  arises  a  tube  which  separates  urine, 
and  which  opens  into  the  body-cavity  in  front,  outside  the 
body  in  the  rear  ;  this  is  the  primitive  kidney  canal. 
10.  Close  by  this  canal,  between  it  and  the  notochord,  develop 
the  rudiments  of  the  sexual  glands  (the  testes  and  ovary), 
in  the  form  of  roundish  cellular  masses,  which  penetrate 
from  the  wall  of  the  body-cavity  to  this  position  (the  un- 
defined boundary  of  the  skin-fibrous  layer  a:id  the  intestinal- 
horous  layer).90 

These  chief,  fundamental,  and  palingenetic  acts  in  the 


282  THE   EVOLUTION   OF  MAN. 

evolution  of  the  individual,  the  assumption  of  which  is 
justified  by  the  comparative  germ-history  of  Vertebrates, 
re-occur  essentially  in  all  branches  of  this  tribe,  though  in 
single  cases  they  are  more  or  less  changed,  or  kenogeneticalh 
modified  In  their  simplest  and  earliest  form,  which  is 
certainly  mainly  palingenetic,  we  yet  find  them  in  the 
Amphioxus;  in  the  Round-mouths  (Cyclostomi),  Fishes,  and 
Amphibia  they  have  already  become  much  changed  and 
vitiated,  kenogenetically  transformed  ;  and  this  is  true 
in  a  much  greater  degree  of  the  three  higher  vertebrate 
classes,  Reptiles,  Birds,  and  Mammals.  In  these  the  gradual 
formation  of  a  very  large  nutritive  yelk  and  of  peculiar 
egg-membranes  has  introduced  so  many  changes,  or 
secondary  kenogenetic  modifications,  that  at  first  sight  it 
is  hardly  possible  to  recognize  the  primary  palingenetic 
incidents  of  evolution. 

In  these,  the  kenogenetic  relation  of  the  germ  to  the 
nutritive  yelk  is  especially  prominent,  and  till  quite  recently 
caused  an  entirely  false  conception  of  the  first  and  most 
important  conditions  of  the  germ  of  the  higher  Vertebrates, 
introducing  many  false  views  as  to  the  Ontogeny  of  these. 
Previously,  the  germ-history  of  the  higher  Vertebrates  was 
universally  based  on  the  view  that  the  first  rudiment  of  the 
germ  is  a  flat  layer-shaped  disc ;  and  for  this  reason  the 
cell  strata  which  compose  the  germ- disc  (also  called  the 
germ-area)  were  called  "  germ-layers."  This  flat  germ-disc 
which  is  at  first  circular,  afterwards  oval,  and  which  in  the 
hen's  egg  we  have  learned  to  call  the  tread  (cicatricula), 
lies  at  a  particular  point  on  the  outer  surface  of  the  large 
globular  nutritive  yelk.  "When  germination  begins,  the  flat 
germ-disc  arches  outwards  and  detaches  its  outer  surface 


THE   "TREAD,"    OR   CICATRICULA. 


283 


from  the  large  yelk-ball  which  lies  beneath  it.     The  flat 
layers  become  tubes,  in  consequence  of  their  edges  inclining 


FIG.  70. — Separation  of  the  disc-shaped  mammalian  germ  from  the 
yelk-sac ;  in  transverse  section  (diagrammatic).  A.  The  germ-disc  (h,hf) 
lies  flat  on  one  side  of  the  intestinal  germ-vesicle  (kb).  B.  The  spinal  tube 
(mr)  appears  in  the  centre  of  the  germ-disc,  and  underneath  t'his  the 
notochord  (ch).  C.  The  intestinal-fibrous  layer  (df)  has  grown  round 
the  intestinal-glandular  layer  (dd).  D.  The  skin-fibrous  layer  (hf)  and  the 
intestinal-fibrous  layer  (df)  separate  in  the  outer  wall ;  the  intestine  (d ) 
begins  to  separate  itself  from  the  navel-vesicle '(nb).  E.  The  intestinal  tube 
(mr)  is  closed ;  the  body-cavity  (c)  begins  to  form.  F.  The  primitive  verte- 
brae (w)  separate  ;  the  intestine  (d)  is  almost  ent'rely  closed.  G.  The 
primitive  vertebrae  (w)  begin  to  grow  •  round  the  spinal  tube  (mr)  and  the 
notochord  (ch)  ;  the  intestine  (d)  is  separate  from  the  navel-vesicle  (nb). 
H.  The  vertebrae  (ir)  have  grown  round  the  spinal  tube  (mr)  and  the  noto- 
chord ;  the  body-cavity  (c)  is  closed,  the  navel-vesicle  has  disappeared.  Tho 
amnion  and  serous  membrane  are  omitted. 

In  all,  the  letters  indicate   the   same  parts  :    li,  horn-plate  ;    mr,  spinel 
tube;  hf,  skin-fibrous  layer;  w,  primitive  vertebrae;  ch,  notochord  ;  c,  body- 
cavity  (cceloma) ;  df,  intestinal-fibrous  layer;  dd,  intestinal -glandular  layer; 
d,  intestinal  cavity  ;  nb,  navel-vesicle. 
21 


284  THE    EVOLUTION    OF   MAN. 

towards  each  other  and  coalescing  (Fig.  70).  The  germ 
growing  at  the  expense  of  the  nutritive  yelk,  the  latter 
continually  becomes  smaller;  it  is  completely  surrounded 
by  the  growth  of  the  germ -layers.  At  a  later  period  the 
remnant  of  the  nutritive  yelk  forms  only  a  small  globular 
sac,  the  yelk-sac,  or  navel-sac  (saccus  vittelinus,  or  vesicula 
umbilicalis,  Fig.  70  nb).  This  is  surrounded  by  the  intes- 
tinal layer,  and  connected  with  the  central  portion  of  the 
intestinal  tube  by  a  thin  stalk,  the  yelk-duct  (ductus 
vitellinus),  and,  in  most  Vertebrates,  is  at  last  completely 
absorbed  by  the  intestinal  tube  (Fig.  70  H}.  The  point  at 
which  this  happens,  and  at  which  the  intestine  finally 
closes,  is  the  intestinal  navel.  In  Mammals,  in  which  the 
remnant  of  the  yelk-sac  remains  outside  and  gradually 
dwindles,  the  yelk-duct  pierces  the  outer  ventral  wall  to  the 
last.  The  navel  cord  parts  at  birth  at  this  point,  which  per- 
manently remains  as  the  navel  (umbilicus)  in  the  outer  skin. 

As  in  the  germ-history  of  the  higher  Vertebrates,  based 
chiefly  on  that  of  the  Chick,  the  distinction  between  the 
germ  (or  formative  yelk)  and  the  nutritive  yelk  (or  yelk 
sac)  has  up  to  the  present  time  been  regarded  as  original, 
the  flat,  leaf-shaped  rudiment  of  the  germ-disc  has  also 
necessarily  been  regarded  as  the  original  germ-form,  and 
the  greatest  weight  has  been  laid  on  the  fact  that  these 
flat  germ-layers  curve,  and  thus  become  hollow  trenches, 
and  that,  by  the  concrescence  of  their  edges,  they  become 
closed  tubes. 

This  view,  which  has  governed  all  past  expos' tions  oi 
the  ge»-m-history  of  the  higher  Vertebrates,  is,  I  am  con- 
vinced, entirely  false.  For  the  Gastrsea  Theory,  the  full 
significance  of  which  now  becomes  evident,  teaches  us  that 


THREE   STAGES   IN    VERTEBRATE    PHYLOGENY. 


285 


the  real  state  of  the  case  is  originally  just  the  opposite. 
The  Gastrula,  in  the  body-wall  of  which  the  two  primary 
germ-layers  appear  from  the  first  as  closed  tubes,  is  the 
original  germ  form  of  all  Vertebrates,  as  of  all  Invertebrate 
animals;  and  the  flat  germ-disc,  with  its  flatly  extended 
geiin-layers,  is  a  later,  secondary  germ-form,  which  arose 
in  consequence  of  the  kenogenetic  formation  of  the  large 
nutritive  yelk,  and  the  consequent  extension  of  the  germ- 
layers  over  the  surface  of  the  latter.91  The  curving  of  these 
germ-layers,  which  actually  occurs,  and  their  coalescence 
into  tubes  is,  therefore,  not  original  and  primary,  but  a  much 
later,  tertiary  incident  of  evolution.  Accordingly,  the  three 
following  stages  of  germ-formation  must  be  distinguished  in 
the  Phylogeny  of  Vertebrates : 


A.  First  Stage  : 
Primary 
(Palingenetic) 
Process  of  Germ- 
formation. 

B.  Second  Stage  : 
Secondary 
(Kenogenetic) 
Process  of  Germ, 
formation. 

C.  Third  Stage  : 
Tertiary 
(Kenogenetic) 
Process  of  Germ- 
formation. 

From    the    first,   the 
perm-layers  form  closed 
tubes. 
No  nutritive  yelk. 

The   perm-layers 
spread   themselves  out 
in  the  form  of   a  leaf, 
in  consequence  of  the 
formation    of    a   large 
yelk-sac  from  the  cen- 
tre   of     the    intestinal 
tube. 

The  germ  -layers  form 
a    flat    germ-disc,    the 
edges  of  which  incline 
toward  each  other,  and 
coalesce  into  »i   closed 
tube. 

If  this  view  is  correct,  and,  as  the  logical  conclusion 
from  the  Gastrsea  Theory,  I  am  obliged  to  believe  it  is  so, 
then  the  explanation  of  the  process  as  at  present  accepted 
must  be  exactly  reversed.  The  yelk-sac  must  no  longer 
be  treated  as  though  it  were  originally  distinct  from  the 


286  THE   EVOLUTION    OF   MAN. 

germ  or  embryo,  but  as  essentially  a  part  of  the  latter,  a 
part  of  its  intestinal  tube.  According  to  this  view,  the 
primitive  intestine  (protogaster')  of  the  Gastrula  of  the 
higher  animals  has  separated,  in  consequence  of  the  keno- 
genetic  formation  of  the  nutritive  yelk,  into  two  different 
parts :  the  after-intestine  (metagaster)  or  the  permanent 
intestinal  canal,  and  the  yelk-sac  or  navel-vesicle. 

If  the  germ-histories  of  the  Amphioxus,  the  Frog,  the 
Chick,  and  the  Rabbit  are  comparatively  studied  (Plates  II., 
III.),  I  am  convinced  that  there  can  be  no  doubt  as  to  the 
correctness  of  this  view,  which  I  have  thus  explained.  In 
the  light  afforded  by  the  Gastnea  Theory  we  shall  regard 
the  structural  proportions  of-  the  Amphioxus  alone  of  all 
Vertebrates,  as  original  and  but  slightly  varied  from  the 
palingenetic  germ-forms.  In  the  Frog  these  proportions  are 
on  the  whole  but  slightly  kenogenetically  altered.  In  the 
Chick,  on  the  contrary,  they  are  very  much  altered,  and 
most  of  all  in  the  Rabbit.  Both  in  the  Bell-gastrula  of  the 
Amphioxus  and  in  the  Hood-gastrula  of  the  Frog,  the  germ- 
layers  are  visible  from  the  first  in  the  form  of  closed  tubes 
(Plate  II.  Fig.  6,  11).  But,  on  the  other  hand,  the  em- 
bryonic Chick  (in  the  freshly-laid,  unincubated  egg)  appears 
in  the  form  of  a  flat,  circular  disc ;  it  was  only  quite 
recently  that  the  true  gastrula-character  of  this  germ-disc 
was  recognized  by  Rauber  and  Goette.74  This  Disc-gastrula 
grows  and  surrounds  the  huge  globular  yelk,  and  the  after- 
intestine  (metagaster}  parts  off  from  the  external  yelk-sac  ; 
in  these  two  processes  all  that  is  diagrammatically  repre- 
sented in  Fig.  70  is  accomplished ;  and  these  are  the  pro- 
cesses which  have  been  regarded  as  main  acts,  though  they 
are  in  reality  only  secondary  acta 


GROWTH   OF   THE   MAMMALIAN    GASTRULA.  287 

In  Mammals  the  corresponding  germinal  processes  are 
very  complex  and  peculiar.  Till  quite  recently  they  were 
entirely  wrongly  explained;  the  recently  published  researches 
of  Eduard  van  Beneden  69  which  placed  them,  for  the  first 
time,  in  a  true  light,  enabled  us  to  bring  them  into  agree- 
ment with  the  principles  of  the  Gastrsea  Theory,  and  to  trace 
their  relation  to  the  germination  of  the  lower  Vertebrates. 
Although  there  is  no  independent  nutritive  yelk,  distinct 
from  the  formative  yelk,  in  the  mammalian  egg,  and 
although  the  cleavage  is  therefore  total,  a  large  yelk-sac 
arises  from  the  embryo  which  is  produced  by  this  cleavage, 
and  the  true  germ  spreads  itself  in  a  layer-like  form  on  the 
surface  of  this  yelk-sac,  as  in  the  case  of  Reptiles  and  Birds, 
the  eggs  of  which  have  a  large  nutritive  yelk  and  undergo 
partial  cleavage.  As  in  the  latter,  the  flat,  leaf-shaped 
germ-disc  of  Mammals  detaches  itself  from  the  yelk-sac, 
its  walls  incline  towards  each  other  and  coalesce  into  tubes. 

This  striking  contradiction  can  only  be  explained  as  a 
consequence  of  very  peculiar,  strange,  kenogenetic  modi- 
fications of  the  germ,  the  causes  of  which  are  not  yet  fully 
explained.  They  are  evidently  connected  with  the  fact 
that  the  ancestors  of  the  viviparous  Mammals  were  Amnion- 
animals,  which  laid  eggs,  and  which  only  gradually  became 
viviparous.  When  the  Hood-gastrula  (Amphigastrula)  of 
the  Mammal  is  complete  (Fig.  71),  it  changes  into  a  large 
globular  vesicle,  filled  with  fluid.  According  to  Van 
Beneden,  this  happens  in  the  following  way :  The  Gastrula- 
mouth  disappears  in  consequence  of  the  entoderm-cell  (o), 
which  formed  the  yelk-plug,  disappearing  into  the  interior, 
to  the  other  cells  of  the  intestinal  layer  (d).  The  mam- 
malian germ  now  forms  a  solid  ball,  consisting  of  a  quantity 


288 


THE   EVOLUTION   OF  MAN. 


of  dark,  multilateral  entoderm-cells  (i),  and  covered  by  a 
light-coloured  globular  membrane,  which  is  composed  of  a 
single  stratum  of  exoderm-cells  (e).  A  transparent  bright- 
coloured  liquid  now  collects  at  a  point  between  the  two 
germ-layers ;  and  this  increases  so  greatly  that  it  expand? 
t-he  exoderm  cellular  membrane  into  a  large  globular  vesicle. 
The  mass  of  entoderm-cells,  forming  a  ball  of  smaller 
diameter,  remains  attached  to  one  point  of  the  exoderm ; 
(according  to  Van  Beneden,.  this  point  is  that  of  the  yelk- 
plug,  o).  The  entoderm  mass  now  becomes  flattened,  first 
assuming  a  hemispherical,  then  a  lentil-shaped,  and  finally 
a  discoidal  form :  this  is  accomplished  by  a  movement 
among  the  cells,  which  spread  themselves  out  in  a  one- 
layered  circular  disc. 


FIG.  71.— Gastrula  of  a  Mam- 
rnal  (Amphigastrula  of  a  Eabbit) 
in  longitudinal  section  through 
the  axis  :  e,  exoderm-cells  (64, 
lighter-coloured  and  smaller)  ;  i, 
entoderm-cells  (32,  darker  and 
larger) ;  d,  central  entoderm-cells, 
occupying  the  primitive  intes- 
tinal cavity ;  o,  peripheric  ento- 
derm-cells, plugging  the  primi- 
tive mouth-opening  (the  yelk- 
plug  in  the  "anus  of  Rusconi"). 


This  vesicular  condition  of  the  mammalian  germ  was 
detected  two  centuries  ago  (1677)  by  Regner  de  Graaf.  He 
discovered  small,  globular,  transparent  vesicles,  \\ith  a 
double  membrane,  lying  free  in  the  uterus  of  a  Rabbit  four 
days  after  impregnation.  But  Graaf's  statement  was  not 
accepted.  It  was  not  till  1827  that  these  vesicles  were 


THE    PROCHORION.  289 

re-discovered  by  Baer ;  those  of  the  Rabbit  were  afterwards 
more  minutely  described  by  Biscboff,  in  1842.  They  may 
be  found  in  the  uterus  (matrix)  of  the  Rabbit,  the  Dog,  and 
other  small  Mammals  within  a  few  days  after  impregnation. 
The  ripe  mammalian  eggs,  having  left  the  ovary,  are  fer- 
tilized, either  here  or  in  the  oviduct,  by  the  active  sperm- 
cells  which  make  their  way  in.92  (On  the  uterus  and 
oviduct  cf.  Chapter  XXV.)  Cleavage  and  gastrulation 
take  place  within  the  oviduct.  Either  while  the  mam- 
malian Gastrula  is  still  in  the  oviduct,  or  after  it  has  entered 
the  uterus,  it  changes  into  the  globular  vesicle  which  is 
represented  in  Fig.  72  (the  surface)  and  in  Fig.  73  (in 


FIG.  72.— Intestinal  germ-vesicle  (Gastrocystis)  of  a  Eabbit  (the  so-called 
"  Germ-vesicle,"  or  vesicula  llastodermica,  of  other  writers)  :  a,  external 
egg-membrane  (chorinri) ;  b,  skin-layer  (exoderma),  forming  the  whole  wall  of 
the  germ-yelk  vesicle;  c,  heap  of  dark  cells,  forming  the  intestinal  layer 
(entoderma^ . 

FIG.  73.— The  same  in  section.  The  letters  as  in  Fig.  72 :  d,  hollow 
space  within  the  intestinal  germ-vesicle- 

section).  The  thick,  external,  structureless  membrane  which 
surrounds  this  is  a  modification  of  the  original  egg-mem- 
brane (zona  pdlucida,  p.  135),  with  the  addition  of  an 


290 


THE   EVOLUTION   OF   MAN. 


albuminous  stratum,  which  has  been  externally  deposited. 
In  future  we  shall  call  this  membrane  the  outer  egg-mem- 
brane, the  primary  chorion  (prochorion,  a).  The  real  wall 
of  the  vesicle,  surrounded  by  this  outer  egg-membrane, 
consists  of  a  simple  layer  of  exoderm-cells  (6),  which  have 
been  regularly  flattened  by  mutual  pressure,  and  most  of 
which  are  hexagonal ;  a  light-coloured  kernel  is  visible 
through  their  finely  granulated  protoplasm  (Fig.  74).  On  a 


FIG.  74. — Four  exoderm-cells  from  the  intestinal  germ-vesicle  of  a 
rabbit. 

FIG.  75. — Two  entoderm-cells  from  the  same. 

point  on  the  inside  of  this  hollow  sphere  lies  a  circular  disc, 
formed  of  darker,  softer,  and  rounder  cells,  of  the  dark 
granulated  entoderm-cells  (Fig.  75). 

The  characteristic  germ-form  in  which  the  developing 
Mammal  now  is  has  usually  been  called  the  "  germ-vesicle  " 
(Keimblase,  Bischoff) ;  the  "  sac-germ  "  (Baer) ;  the  "  vesi- 
cular embryo,"  or  the  "  germ-membrane  vesicle  "  (vesicula 
blastodermica,  or,  briefly,  blastosphcera}.  The  wall  of  the 
hollow  sphere,  consisting  of  a  single  cell-stratum,  was  called 
the  "  germ-membrane,"  or  blastoderm,  and  it  was  supposed 
to  be  equivalent  to  the  cell-stratum,  called  by  the  same 
name,  which  forms  the  wall  of  the  true  germ-membrane 
vcsiclo  (Blastula)  of  the  Amphioxus  (Plate  II.  Fig.  4),  and 


THE   INTESTINAL   GERM-VESICLE.  29 1 

of  many  Invertebrate  Animals  (e.g.  of  the  Monoxenia,  Fig. 
22,  F,  G).  This  true  germ-membrane  vesicle  has,  up  to  the 
present  time,  been  universally  regarded  as  homologous  with 
the  germ-vesicle  of  Mammals.  It  is  not  so,  however.  The 
so-called  "germ-vesicle"  of  Mammals  and  the  true  germ- 
membrane  vesicle  of  the  Amphioxus  and  of  many  Inverte- 
brates are  entirely  different  germ-forms.  The  latter  (the 
Blastula)  precedes  gastrulation.  The  former  (vesicula 
Uastodermica)  follows  gastrulation.  The  globular  wall  of 
the  blastula  is  a  true  germ-membrane  (Blastoderma),  and 
consists  entirely  of  cells  of  one  sort  (blastoderm-cells) ;  it 
is  not  yet  specialized  into  the  two  primary  germ-layers. 
On  the  other  hand,  the  globular  wall  of  the  mammalian 
"  germ-vesicle "  is  the  specialized  skin-layer  (exoderma], 
and  a  circular  disc  of  entirely  different  cells  lies  at  a  point 
on  the  inside  of  this ;  this  disc  is  the  intestinal  layer 
(entoderma).  The  spherical  cavity,  rilled  with  clear  liquid, 
in  the  interior  of  the  blastula,  is  the  cleavage-cavity.  On 
the  other  hand,  the  similar  cavity  in  the  interior  of  the 
mammalian  germ-vesicle  is  the  yelk-sac  cavity,  which  is 
joined  on  to  the  developing  intestinal  cavity. 

On  all  these  grounds,  which  have  been  very  recently 
brought  to  light  by  the  researches  of  Van  Beneden,  it  is 
very  necessary  to  recognize  the  secondary  "intestinal  germ- 
vesicle  "  of  Mammals  (Gastrocystis)  as  a  peculiar  germ-form, 
occurring  only  in  this  class  of  animals,  and  as  quite  distinct 
from  the  "  germ-membrane  vesicle  "  (Blastula)  of  the  Am- 
phioxus and  of  the  Invertebrates.  The  wall  of  this  mam- 
malian "  intestinal  germ- vesicle  "  consists  of  two  distinct 
parts.  Far  the  larger  part  is  one-layered,  and  is  formed  by 
the  exoderm.  For  the  smaller  part,  the  circular  disc,  which 


292  THE   EVOLUTION   OF   MAN. 

is  formed  of  both  primary  germ-layers,  we  will  adopt  Van 
Beneden's  name,  and  call  it  the  intestinal  germ-disc  (Gas- 
trodiscus). 

The  small,  circular,  dull  whitish  spot  which  lies  at  a 
particular  point  on  the  outer  surface  of  the  bright-coloured, 
transparent,  and  spherical  "  intestinal  germ-vesicle,"  and 
which  is  the  "  intestinal  germ-disc  "  (Gastrodiscius),  has  long 
been  known  to  naturalists,  and  was  compared  with  the 
"  germ-disc  "  of  Reptiles  and  Birds.  Sometimes,  therefore,  it 
was  called  the  "  germ-disc  "  (discus  blastodermicus),  some- 
times the  "  embryonic  spot "  (tache  embryonnaire),  but 
more  usually  the  germ-area  (area  germinativa).  The 
further  evolution  of  the  germ  proceeds  especially  from  this 
germ-area.  On  the  other  hand,  the  greater  part  of  the 
intestinal-germ-vesicle  of  Mammals  is  not  directly  employed 
in  the  formation  of  the  future  body,  but  in  the  formation  of 
the  transitory  "  navel- vesicle."  The  embryo-body  pinches 
itself  off  from  the  latter  more  and  more,  in  proportion  as 
it  grows  and  develops  at  the  expense  of  the  latter ;  the 
two  become  no  longer  connected  except  by  the  yelk-duct 
(the  stalk  of  the  yelk-sac) ;  and  the  latter  forms  the  indirect 
communication  between  the  cavity  of  the  navel-vesicle  and 
the  intestinal  cavity  in  the  course  of  development  (Fig.  70). 

The  germ-area,  or  the  intestinal  germ-disc  of  Mammals, 
originally  consists,  like  the  germ-disc  of  Birds,  merely  of 
the  two  primary  germ-layers,  each  of  which  is  formed  of  a 
single  cell-stratum.  Soon,  however,  a  third  cell-stratum,  the 
rudiment  of  the  middle  fibrous  layer  (mesodermq),  appeal's 
in  the  middle  of  the  circular  disc,  between  the  two  earlier 
strata.  According  to  most  observers,  the  mesoderm  arises 
troni  the  inner  primary  germ-layer ;  according  to  othere,  on 


GERM-AREA.  2Q3 

the  contrary,  it  arises  from  the  outer  of  the  two  ; 93  both  are 
probably  concerned  in  the  process.  The  middle  of  the 
germ-area,  or  germ-disc,  now  consists  of  three  germ-layers, 
while  the  circular  rim  consists  of  two ;  the  rest  of  the  wall 
of  the  intestinal  germ-vesicle  consists  only  of  a  single  germ- 
layer,  the  outer.  But  this  wall  also  now  becomes  two-layered. 


FIG.  76. — Section  through  the  germ-area  of  a  Mammal,  at  right  angles 
to  the  surface  (diagrammatic)  :  e,  exoderm  (the  simple  cell-stratum  of  the 
skin-layer)  ;  m,  mesoderm  (the  several  cell-stratum  of  the  middle  layer)  ; 
i,  entoderm  (the  simple  cell-stratum  of  the  intestinal  layer)  ;  k,  hollow 
cavity  in  the  intestinal  germ-vesicle. 

While,  in  the  centre  of  the  germ-area,  the  fibrous  layer 
becomes  greatly  thickened,  in  consequence  of  cell-growth, 
the  inner  germ-layer  simultaneously  extends  and  grows  in 
all  directions  from  the  edge  of  the  disc.  Everywhere  closely 
applied  to  the  outer  germ-layer,  it  completely  overgrows  the 
inner  surface  of  the  latter ;  it  covers  first  the  upper,  and 
then  the  lower  hemisphere  of  the  inner  surface,  and  finally 
closes  in  the  centre  of  the  latter.  (Of.  Fig.  77-81.)  The 
whole  wall  of  the  intestinal  germ-vesicle  now,  therefore, 
consists  of  two  cell-strata:  the  exoderm  without,  the  entoderm 
within.  In  the  centre  only  of  the  circular  germ-disc,  which, 
in  consequence  of  the  excessive  growth  of  the  middle 
layer,  continually  increases  in  thickness,  this  germ-disc 
consists  of  all  three  germ-layers.  Simultaneously,  small 
structureless  knots,  or  warts,  secrete  themselves  on  the 
surface  of  the  outer  egg-membrane  (prochoriori),  which 


294 


THE   EVOLUTION   OF   MAN. 


Fro.  77.— Egg  from  the  uterus  of  a  Rabbit  (4  mm.  in  diameter).  The 
perm -membrane  vesicle  (6)  has  slightly  retreated  from  the  smooth  outer 
egg-membrane  (prochorion,  a).  The  circular  germ-area  (c)  is  visible  in  tho 
centre  of  the  germ-membrane,  and  at  the  edge  of  the  former  (at  d)  the  inner 


DEVELOPMENT   OF   MESODERM.  2Q$ 

stratum  of  the  germ-vesicle  is  already  beginning  to  extend.  (Fig.  77-81, 
after  Bisclioff.) 

FIG.  78. — The  same,  poen  from  one  side.     The  letters  as  in  Fig.  77. 

FIG.  79. — Kgg  from  the  uterus  of  a  Rabbit  (6  mm.  in  diameter).  The 
germ -membrane  is  already  to  a  great  exieut  double -lave  red  (b).  The  outer 
egg-membrane  (prochorwn)  becomes  knotty,  or  wai"ty  (a). 

FIG.  80. — The  same,  seen  from  one  side.     The  letters  as  in  Fig.  79. 

FIG.  81. — Egg  from,  the  uterus  of  a  Rabbit  (8  mm.  in  diameter).  Nearly 
tlie  whole  of  the  germ-membrane  ve.-icle  is  already  double- layered  (b) ;  only 
below  (at  d)  there  is  still  only  one  layer. 

has  raised  itself  from  the  intestinal  germ-vesicle  (Fig. 
79-81  a). 

We  need  not  at  present  pay  any  attention  either  to  this 
outer  egg-membrane  (prochorion)  or  to  the  larger  portion  of 
the  germ- vesicle,  and  may  turn  our  full  attention  to  the 
germ-area  (or  germ-disc).  For  it  is  in  this  part  alone  that 
the  important  modifications  which  result  in  the  specializa- 
tion of  the  earliest  organs  first  appear.  In  this  respect  it  is 
quite  immaterial  whether  we  examine  the  germ-area  of  a 
Mammal  (e.g.  a  Rabbit),  the  germ-disc  of  a  Bird  or  a  Reptile 
(e.g.  a  Lizard  or  a  Tortoise).  For  in  all  members  of  the  three 
higher  vertebrate  classes,  all  called  Amnion-animals,  the 
germinal  processes  which  immediately  follow  are  essentially 
alike.  In  this  respect  Man  is  like  the  Rabbit,  the  Dog,  the 
Ox,  etc. ;  and  in  all  these  Mammals  the  germ-area  undergoes 
essentially  the  same  modifications  as  in  Birds  and  Reptiles. 
It  is  in  the  Chick  that  these  have  been  chiefly  and  most 
accurately  traced,  for  any  requisite  number  of  incubated 
hen's  eggs,  in  all  stages,  can  always  be  obtained.  Within 
a  few  hours  from  the  beginning  of  incubation  the  cir- 
cular germ-disc  of  the  Chick  also  passes  from  a  two- 
layered  to  a  three-layered  stage,  in  consequence  of  the 
development  of  the  mesoderm  between  the  exoderm  and 
the  entoderm. 


THE   EVOLUTION   OF   MAN. 


The  first  modification  of  the  discoidal,  three-layered 
germ-area  consists  in  the  fact  that  the  cells  round  its  edge 
increase  more  rapidly,  and  accumulate  dark  granules  in  their 
protoplasm.  In  this  way  a  darker  ring  is  formed,  which  is 


Fro.  82. — Circular  germ.area  of  a  Rabbit,  distinguished  into  the  central, 
1'ght-coloured  germ-area  (area  pellucida),  and  the  peripheric  dark  germ-area 
(a.  opaca).  As  it  makes  itself  visible  through  the  dark  part,  the  area 
pellucida  appears  the  darker. 

FIG.  83. — Oval  germ-area  (a.  germinativa).  The  dull  whitish  area  opacc, 
appears  on  the  outside. 

more  or  less  distinctly  marked  off  from  the  lighter  centre  of 
the  germ-disc  (Fig.  82).  The  latter  we  shall  in  future  call 
the  light  germ-area  (area  pellucida} ;  the  darker  ring  we 
shall  call  the  dark  germ -area  (area  opaca}.  (In  reflected 
light,  as  in  Fig.  82-84,  it  appears  reversed  ;  the  light  germ- 
area  appears  dark,  because  the  dark  ground  makes  itself 
seen  from  below ;  the  dark  germ-area  appears  lighter  in 
comparison.)  The  circular  form  of  the  germ-area  changes 
to  an  elliptic,  and  immediately  afterwards  to  an  oval  form 
(Fig.  83).  One  end  appears  broader  and  more  abruptly 


THE   GERM-SHIELD. 


297 


rounded  off,  the  other  is  smaller  and  more  pointed;  the 
former  represents  the  hinder  portion  of  the  future  body. 
The  characteristic  bilateral  form  of  the  body,  the  distinc- 
tion between  anterior  and  posterior,  between  right  and 
left,  is  thus  already  indicated. 

In  the  centre  of  the  light  germ-area  a  dull-coloured^ 
large,  oval  spot  now  appears ;  at  first  it  is  very  delicate  and 
hardly  noticeable,  it  soon  however  becomes  more  sharply 
distinguished,  and  presently  appears  as  an  oval  shield, 
surrounded  by  two  rings  (Fig.  84).  The  inner,  lighter  ring 

FIG.  81.  —  Gsrm-area  or 
germ-disc  of  a  Rabbit  (about 
ten  times  magnified).  As 
the  delicate,  half-transparent 
germ-disc  lies  on  black 
ground,  the  light  germ-area 
appears  as  a  darker  ring,  the 
dark  germ-area  (situated  on 
the  outside),  on  the  contrary, 
as  a  white  ring.  The  oval 
germ-shield,  situated  in  the 
centre,  also  appears  whitish  ; 
along  its  axis  the  dark  spinal 
furrow  is  already  visible. 
(After  Bischoff.) 

is  the  remnant  of  the 
light  germ-area ;  the 
outer,  darker  ring  is 
the  dark  germ-area;  but  the  dull-coloured  shield-shaped 
spot  itself  is  the  first  rudiment  of  the  dorsal  portion 
of  the  embryo.  We  will  call  it  briefly  the  "  germ-shield " 
(notaspis).9*  Remak  called  it  the  "  double  shield,"  because 
it  arises  from  a  shield-shaped  thickening  of  the  outer  and  the 
middle  germ-layers.  In  most  books  this  germ-shield  is 


298  THE   EVOLUTION   OF   MAN. 

spoken  of  as  the  "first  germ-rudiment  or  embryonic  rudi- 
ment," as  the  "  primitive  germ,"  or  "  the  first  trace  of  the 
embryo."  But  these  designations,  which  are  based  on  the 
authority  of  Baer  and  Bischoff,  are  incorrect.  For  in  reality 
the  germ  or  embiyo  already  exists  in  the  parent-cell,  in  the 
Gastrula,  and  in  all  the  subsequent  germ-stages.  The  germ- 
shield  is  merely  the  earliest  rudiment  of  that  dorsal  part 
which  first  becomes  defined. 


FIG.  85.— Germ-area  or  grrm-disc  of  a  Rabbi  b,  with  a  sole -shaped  germ- 
shield  (about  ten  times  enlarged).  The  light,  circular  tract  (d)  is  the  dark 
area  (a.  opaca).  The  light  area  (a.  pellucida)  (c)  is  lyre-shaped,  as  is  the 
germ-shield  itself  (6).  Along  its  axis  the  dorsal  furrow  or  spinal  furrow  (a) 
is  seen.  (After  Bischoff.) 

FIG.  86. — Sole-shaped  germ-shield  of  a  Dog  ("double  shield"  of  Remak, 
"embryo-rudiment  "  of  other  authors)  In  the  centre  is  the  dorsal  furrow; 
on  either  side  are  the  spinal  swellings,  or  medullary  swellings. 

FlG.  87. — Sole-shaped  germ-shield  of  Chick. 

After  the  oval  germ-shield  has  become  distinctly  defined, 
hi  the  centre  of  the  light  germ-area,  along  its  central  line 
a  delicate,  white  streak  appears,  which  soon  becomes  pro- 


THE   PRIMITIVE   STREAK.  299 

miment;  this  is  the  "primitive  streak"  of  Baer,  the  "axial 
plate  "  of  Remak.  This  phenomenon  is  due  to  the  fact  that 
the  upper  and  middle  germ-layers  coalesce  along  their 
central  lines,  thus  forming  the  axis-band  at  this  point. 
(Of.  Fig.  88,  89.)  In  the  centre  of  the  primitive  streak  an 
even,  dark  line,  the  so-called  primitive  groove,  becomes 
denned  (Fig.  84,  85,  a).  This  separates  the  germ-shield  into 
two  symmetrical  halves,  a  right  and  a  left  half.  While  the 
primitive  groove  deepens,  the  oval  germ-area  (a.  germina- 
tiva)  resumes  its  earlier  circular  form. 

The  germ-shield,  on  the  other  hand,  leaves  its  oval 
form  and  assumes  the  so-called  lyre-shape,  or  sole-shape. 
Its  elliptical  leaf-shaped  body  becomes  somewhat  pinched 
in  the  middle,  while  its  anterior  and  posterior  ends  become 
somewhat  enlarged  (Fig.  85).  This  very  characteristic 
shape,  which  is  most  aptly  compared  to  the  sole  of  a  shoe, 
a  violin,  or  a  lyre,  is  retained  for  some  time  by  the  embryo 
of  the  Mammal  (Fig.  86,  87),  and  also  by  that  of  the  Bird 
and  the  Reptile.  The  human  germ-shield  assumes  this  sole- 
form  as  early  as  the  second  week  of  its  development. 
Towards  the  end  of  that  week  its  length  is  about  two 
millimetres. 

We  will  now  leave  the  peripheric  part  of  the  germ- 
area,  for  its  changes  are  only  interesting  to  us  at  a  much 
later  period,  and  we  will  give  our  whole  attention  to 
the  sole-shaped  germ-shield,  from  which  the  further  evolu- 
tion of  the  body  directly  proceeds.  In  order  correctly  to 
understand  this,  we  must  employ  a  method  which  was  first 
turned  to  full  account  by  Remak,  viz.,  that  of  viewing 
sections  made  from  right  to  left  perpendicularly  through  the 

thin  disc  of  the  germ-shield.     It  is  only  by  very  carefully 

22 


300 


THE    EVOLUTION    OF   MAN. 


studying  these  sections,  one  by  one,  in  every  stage  of  the 
evolution,  that  it  is  possible  fully  to  understand  the  pro- 
cesses by  which  the  exceedingly  complex  structure  of  the 
vertebrate  body  is  developed  from  the  simple  leaf-shaped 
germ-shield. 

If  we  now  make  a  perpendicular  section  through  the 
sole-shaped  germ-shield  (Fig.  86,  87),  the  first  thing  we 
notice  is  the  difference  between  the  three  germ-layers,  as 
they  lie  one  over  the  other  (Fig.  88).  The  germ-shield  con- 
sists, as  it  were,  of  three  shoe-soles  overlying  each  other. 
The  undermost,  or  innermost,  of  these  (the  intestinal -gland- 
ular layer)  is  the  thinnest  stratum,  and  consists  of  a  single 
layer  of  cells  (Fig.  88  d).  The  middle  of  these  shoe-shaped 
bodies  (the  mesoderm)  is  considerably  thicker  and  more  or 
less  evidently  appears  to  be  composed  of  two  closely  con- 
nected layers.  The  third  and  uppermost,  or  outermost  sole- 


Fro.  88. — Transverse  section  through  the  germ-disc  of  a  Chick  (a  few 
hours  after  the  beginning  of  incubation)  :  h,  skin-sensory  layer;  m,  skin- 
fibrous  layer ;  /,  intestinal-fibrous  layer  (the  two  latter  are  united  into  the 
middle-layer,  or  mesoderm)  ;  d,  intestinal-glandular  layer.  All  the  four 
secondary  germ-layers  have  coalesced  in  the  middle  and  from  the  thick 
axis-band  (xy~)  •  n,  first  trace  of  the  primitive  groove ;  it,  region  of  the 
future  primitive  kidney  rudiment.  (After  Waldeyer.) 


shaped  body  (A),  is  the  skin-sensory  layer,  and  consists  of 
smaller  and  lighter-coloured  cells.  In  the  middle  of  the 
transverse  section,  along  a  considerable  part  of  the  longi- 


INTERCHANGE  OF  CELLS  BETWEEN  THE  TWO  GERM-LAYERS.  3OI 

tudinal  axis  of  the  sole,  all  three  soles  coalesce,  and  here  form 
the  thick  axial  band  (Fig.  88,  xy).  This  coalescence  is  very 
significant.  It  causes  an  exchange  of  cells  between  the 
primary  germ-layers.  These  cells  move,  alter  their  position, 
and  multiply,  so  that  exoderm-cells  penetrate  among  the 
entoderm-cells,  and  entoderm-cells  among  those  of  the  exo- 
derm.  The  middle  layer,  or  mesoderm,  therefore,  contains 
cells  from  both  of  the  two  primary  germ-layers.  Even 
if  Remak's  explanation,  according  to  which  the  mesoderm 
is  originally  split  off  from  the  entoderm,  is  correct,  in 
consequence  of  the  coalescence  at  the  central  point,  exo- 
derm  cells  may  also  afterwards  make  their  way  into  the 
mesoderm.  The  fibrous  layer  indeed  soon  plainly  shows 
that  it  is  composed  of  two  different  strata ;  the  outer, 
which,  phylogenetically,  must  be  referred  to  the  skin-layer, 
and  the  inner,  which  must  be  referred  to  the  intestinal 
layer.  The  outer  is  the  rudiment  of  the  skin-fibrous  layer 
(Fig.  88,  m,  89,  m) ;  the  inner  becomes  the  intestinal-fibrous 
layer  (Fig.  88,  /,  89,  /).  Soon  after  the  coalescence  of  the 
germ-layers  in  the  axial  portion  of  the  germ-shield  has 
taken  place,  and  the  cells  have  been  exchanged,  the  small 
rectilineal  primitive  groove  (Fig.  89,  n)  becomes  visible  in 


FIG.  89.— Transverse  section  through  the  germ-shield  of  a  Chick  (in  a 
stage  rather  later  than  in  Fig.  88).  The  letters  indicate  the  same  parts  as 
in  Fig.  88.  In  the  middle  of  the  axis-band  (y)  the  chorda  dorsalis,  or  noto- 
chcrd,  becomes  denned  (»).  (After  Waldeyer.) 


302  THE   EVOLUTION    OF    MAN. 

the  central  line  of  the  outer  surface.  On  each  side  of  this, 
the  dorsal  swellings  rise  in  the  form  of  low  ridges.  In  the 
centre  of  the  lower  side  of  the  primitive  groove  a  cylin- 
drical band  separates  itself  from  the  cell-mass  of  the  thick 
axis-band ;  this,  which  in  transverse  section  appears 
roundish,  is  the  first  rudiment  of  the  notochord  (chorda 
dorsalis,  x).  The  four  secondary  layers  separate  more  and 
more  distinctly.  The  intestinal-fibrous  layer  (/)  appears 
as  the  product  of  the  intestinal-glandular  layer  (d),  and 
distinct  from  the  skin-fibrous  layer  (m),  which  arises  from 
the  skin-sensory  layer  (h). 


FIG.  90. — Transverse  section  through  the  germ-shield  of  an  incubated 
Chick  (about  the  end  of  the  first  day)  ;  about  100  times  the  natural  size. 
The  skin-sensory  layer  (the  outer  gerin-layer)  separates  into  two  different 
parts  :  (1)  the  thinner,  peripheric  horn-plate  (h),  from  which  the  outer  skin 
with  its  appendages  arises;  (2)  the  thicker,  axial  spinal  plate  (m),  which 
gives  rise  to  the  spinal  tube  (tubus  medullaris) ;  this  originates  from  the 
dorsal  furrow  (-B/),  the  deepest  part  of  which  forms  the  primitive  groove 
(Pv).  The  boundaries  between  the  spinal  plate  (m)  and  the  horn-plate  (/<). 
form  the  prominent,  parallel  dorsal  swellings.  The  middle  germ-layer,  the 
compound  fibrous  layer  (the  "  motor-germinative  "),  is  already  distinguished 
into  the  notochord  (c/i)  and  the  two  side-layers  (sp).  The  inner  portion  of 
these  side-plates  soon  becomes  defined  as  the  primitive  vertebral  band 
(uwp).  The  tiny  fissure  in  the  side-plates  is  the  first  rudiment  of  the 
future  body-cavity  (uwh).  The  inner  germ-layer  (the  intestinal-glandular 
layer)  (d  d)  is  not  yet  modified.  (After  Kolliker.) 

The  primitive  groove  (Fig.  90  Pv}  soon  becomes  con- 
siderably deeper  and  so  fashioned  as  to  constitute  the  bed 
of  the  broader  spinal  furrow  (medullary  or  dorsal  furrow) 


THE   SIDE-LAYERS.  303 

(-S/).  On  both  sides  of  this  rise  the  two  parallel  dorsal 
swellings,  or  spinal  swellings  (m).  At  the  same  time  the 
central  notochord,  or  chorda  dorsalis  (Fig.  90  cfi),  separates 
entirely  and  definitely  from  the  two  lateral  portions  of  the 
mesoderm.  These  we  will  henceforth  regard  as  side-layers 
(sp)  in  reference  to  the  axial  chord.  They  are  usually 
called  side-plates.  In  the  middle  of  each  of  these  side- 
layers  a  horizontal  fissure  appears,  where  the  upper  or  outer 
skin-fibrous  layer  separates  from  the  lower  or  inner  intestinal- 
fibrous  layer.  This  fissure  (Fig.  90  uwh)  is  very  significant, 
for  it  represents  the  first  rudiment  of  the  future  body- 
cavity  (c&loma).  (Of.  Plate  IV.  Fig.  2,  c  and  3,  c.) 

In  speaking  of  these  side-layers,  which  are  usually 
called  "  side-plates,"  I  would  say  a  word  or  two  about 
those  figurative  expressions  "  layers "  and  "  plates,"  which 
have  been  universally  employed  since  Baer's  time.  The 
"  layers  "  (lamince),  as  well  as  the  "  plates  "  (lamellae),  are 
leaf-like  or  plate-shaped  bodies  originally  consisting  of  a 
single  homogeneous  cellular  stratum,  or  of  several  lying  one 
above  the  other,  and  constituting  the  first  basis  of  the 
organic  systems  and  of  the  organs  of  the  body.  But  the 
language  of  Ontogeny  distinguishes  considerably  between 
a  layer,  or  leaf  (lamina'),  and  a  plate  (lamella).  The  first 
and  eldest  cell-layers  of  the  germ,  which  overspread  the 
whole  germ,  and  form  the  first  basis  of  whole  organ-systems, 
are  layers,  or  leaves  (lamince).  On  the  other  hand,  the 
term  plates  (lamellae)  is  applied  to  separate  portions  of  the 
layers,  or  leaves,  and  to  the  cellular  strata  produced  from 
the  latter,  which  only  belong  to  a  part  of  the  germ  and 
serve  to  form  single  organs  of  variable  size. 

Of   course    this    distinction   is   by   no    means   sharply 


304  THE   EVOLUTION   OF   MAN. 

drawn :  thus,  for  instance,  the  two  middle,  secondary  germ- 
layers  are  usually  called  the  skin-fibre  plate  and  the  intes- 
tinal-fibre plate  (instead  of  layers,  or  leaves).  Conversely, 
the  horn-plate  (which  is  a  portion  of  the  skin-sensory 
layer)  is  usually  called  the  horn-layer,  or  leaf.  As  far  as 
possible  we  shall,  however,  maintain  this  important  distinc- 
tion :  we  shall  only  use  the  term  layers,  or  leaves,  of  the 
two  primary,  and  the  four  secondary  germ-layers ;  naturally, 
however,  we  must  speak  of  the  side-plates  as  side-layers,  or 
leaves,  as  they  first  originate  by  a  coalescence  of  the  two 
primary  germ-layers.  On  the  other  hand,  we  shall  speak 
of  the  so-called  horn-layer  and  of  all  the  layer-like  rudi- 
mentary organs,  which  are  split  off  or  differentiated  from 
the  four  layers,  or  leaves,  as  plates;  e.g.  the  muscle-plate, 
etc. 

After  the  chorda  has  entirely  separated  from  the  two 
side-layers,  a  portion,  in  the  shape  of  a  long,  thick  cord, 
breaks  off,  in  the  posterior  portion  of  the  germ-shield,  from 
the  inner  edge  of  each  of  the  side-layers  (Fig.  90,  uwp,  91,  u). 
We  will  call  this  the  primitive  vertebral  plate,  or  better, 
the  primitive  vertebral  cord,  for  it  afterwards  develops  into 


FIG.  91.— Transverse  section  through  the  germ-shield  of  a  Chick  (at  the 
end  of  the  first  day),  rather  more  developed  than  in  Fig.  90 ;  about  twenty 
times  the  natural  size.  The  two  edges  of  the  spinal  plate  (m),  which,  as 
spinal  swellings  (to),  separate  the  latter  plate  from  the  horn-plate  (h),  incline 
towards  each  other.  On  both  sides  of  the  notochord  (cTi)  the  inner  portion 
of  the  side-layers  (u)  has  separated  itself  as  a  primitive  vertebral  band 
from  the  outer  portion.  The  intestinal-glandular  layer  (d)  is  not  yet 
modified.  (After  Remak.) 


THE   PRIMITIVE    VERTEBRAL   CORD.  305 

the  primitive  vertebrae  and  the  neighbouring  parts.  It  forms 
the  first  rudiment  ot  the  individual  segments  of  the  verte- 
bral column,  the  primitive  vertebras.  At  a  later  period  these 
primitive  vertebrae  become  very  closely  connected  with  the 
chorda  dor  sails  which  they  surround,  and  this  whole  axis- 
mass  then  develops  into  the  vertebral  column,  which  is 
afterwards  articulated  in  so  many  complex  ways.  The 
peripheral  parts  of  the  two  side-layers,  which  remain  after 
the  separation  of  the  primitive  vertebral  cord,  are  hence- 
forth called  the  side-plates  (lamellce),  the  term  being  thus 
used  iu  its  restricted  sense.  They  develop  into  the  two 
fibrous  layers,  which  have  already  been  mentioned.  In  the 
anterior  half  of  the  germ-shield,  representing  the  future 
head,  there  is  no  separation  between  the  inner  primitive 
vertebral  mass  and  the  outer  side-layers. 

During  these  processes,  this  intestinal-glandular  layer, 
the  inner  germ-layer,  remains  quite  unaltered;  no  separations 
are  to  be  seen  in  it  (Figs.  90,  dd,  91,  d).  The  changes,  there- 
fore, which  take  place  at  this  period  in  the  skin-sensory 
layer,  the  outer  germ -layer,  are  all  the  more  remarkable.  The 
continuous  elevation  and  growth  of  the  dorsal  swellings  tends 
to  make  the  upper,  free  margins  of  these  prominent  ridges 
incline  towards  each  other,  and  as  they  continually  ap- 
proach each  other  (Fig.  91,  w),  they  finally  coalesce.  Thus 
the  open  dorsal  furrow,  the  separation  at  the  top  of  which 
grows  narrower  and  narrower,  is  transformed  into  a  closed 
cylindrical  tube  (Fig.  92,  mr).  This"  tube  is  of  the  greatest 
importance,  for  it  is  the  first  basis  of  the  central  nervous 
system — the  brain  and  spinal  cord.  This  rudiment  is  called 
the  medullary  tube  (tubus  medullaris*).  Formerly  this  fact 
\vas  regarded  with  wonder  as  an  inexplicable  enigma,  but 


306  THE    EVOLUTION    OF   MAN. 

the  Theory  of  Descent  explains  it  as  but  a  perfectly  natural 
process.  It  is  quite  natural  that  the  central  nervous  system, 
the  organ  by  which  all  intercourse  with  the  outer,  world, 
all  mental  activities,  and  all  sensory  perception  are  accom- 
plished, should  be  developed  by  detachment  from  the  outer 
skin  (epidermis).  At  a  later  stage  the  medullary  tube 
separates  entirely  from  the  outer  germ-layer,  is  surrounded 
by  the  primitive  vertebrae,  and  is  forced  inwards.  From  this 
time,  the  remaining  portion  of  the  skin-sensory  layer  (Fig. 
92  Ji),  is  called  the  horn-plate  or  "  horn-layer,"  because  the 


FIG.  92. — Transverse  section  through  the  germ-shield  of  a  Chick  (second 
day  of  incubation)  ;  about  100  times  the  natural  size.  In  the  outer  germ- 
layer,  the  axial  dorsal  furrow,  having  completely  closed,  forms  the  spinal 
tube  (mr),  and  has  pinched  itself  off  from  the  horn-plate  (h).  In  the  middle 
germ-layer,  the  axial  notochord  (ch~)  is  entirely  separated  from  the  primitive 
vertebral  bands  (uw},  in  the  interior  of  which  a  transitory  cavity  (uwh) 
afterwards  forms.  The  side-layers  have  split  into  the  outer  skin-fibrous- 
layer  (hpl)  and  the  inner  intestinal-fibrous  layer  (df),  the  two  being  still 
connected  by  the  middle  plates  (mp).  The  fissure  (sp)  between  the  two  is 
the  first  rudiment  of  the  body-cavity  (ccvloma).  In  the  gap  between  the 
primitive  vertebral  bands  and  the  side-layers,  on  either  side,  is  the  primitive 
kidney  (ung),  and  on  the  inside  the  primitive  artery  (oo).  (After  Kolliker.) 

outer  skin  (epidermis),  with  its  horny  appendages — nails, 
hair,  etc.— develops  from  it.     (Cf.  Plates  IV.  and  V.) 

At  a  very  early  period,  in  addition  to  the  central  nervous 
system  another  or  wholly  different  organ  is  seen  to  arise 
from  the  outer  skin  ;  this  is  the  primitive  kidney,  which 


THREEFOLD   SEPARATION   OF   THE   MESODERM.  307 

accomplishes  the  excretory  functions  of  the  body,  and  se- 
cretes the  urine  of  the  embryo.  The  primitive  kidney 
originally  consists  of  an  entirely  simple,  tubular,  elongated 
passage,  a  straight  duct  situated  on  each  side  of  the  ventral 
aspect  of  the  primitive  vertebral  cord,  running  from  an 
anterior  to  a  posterior  direction  (Fig.  92,  ung].  It 
apparently  arises  from  the  horn-plate,  and  at  the  side 
of  the  medullary  tube  (spinal  tube),  in  the  space  between 
the  primitive  vertebral  cord  and  the  side-plates.  Even 
while  the  medullary  tube  is  separating  from  the  horn-layer, 
the  primitive  kidney  is  visible  in  this  gap.  Some  authors, 
however,  hold  that  the  first  rudiment  of  the  primitive 
kidney  is  not  furnished  by  the  skin-sensory  layer,  but  by 
the  skin-fibrous  layer. 

While  the  skin-sensory  layer  is  thus  splitting  up  into 
the  horn-plate,  the  spinal  tube,  and  the  primitive  kidneys, 
the  mesoderm,  or  fibrous  layer,  also  separates  into  three  por- 
tions, viz.:  (1)  the  notochord  in  the  central  line  of 'the  germ- 
shield  (Fig.  92,  ch] ;  (2)  the  primitive  vertebral  bands  on 
each  side  of  the  notochord  (uw) ;  and  (3)  the  side-layers  which 
separate  from  the  exterior  of  primitive  vertebral  bands. 
These  side-layers  still  show  the  original  separation  of  the 
middle  germ-layer  into  the  outer  skin-muscle  layer  (or  skin- 
fibrous  layer,  hpl),  and  the  inner  intestinal-muscle  layer 
(or  intestinal-fibrous  layer,  df).  The  point  of  union  of  the 
two  fibrous  layers  is  called  the  middle  plate,  or  mesentery- 
plate  (mp).  The  narrow  fissure  (sp),  or  empty  space 
which  arises  between  the  two  fibrous  layers,  is  the  first 
rudiment  of  the  body-cavity  (cceloma),  the  great  visceral 
cavity,  in  which  the  heart,  lungs,  intestines,  etc.,  are  after- 
wards situated.  In  Mammals  this  is  separated,  at  a  later 


3O8  THE   EVOLUTION   OF   MAN. 

period,  into  two  distinct  cavities  by  the  formation  of  the 
diaphragm ;  these  are  the  chest,  or  thoracic  cavity,  and  the 
abdominal  cavity.  Immediately  below  the  mesentery -plate, 
in  the  gap  between  the  intestinal-glandular  layer,  the  in- 
testinal-fibrous layer,  and  the  primitive  vertebral  bands, 
another  organ  appears  at  an  early  stage,  in  the  form  of  a 
tube  with  a  thin  wall  (Fig.  92,  ao).  This  is  the  first  rudi- 
ment of  a  large  blood-vessel,  the  primitive  artery,  or  aorta. 
It  arises  by  fission  from  the  intestinal-fibrous  layer. 

During  these  processes  the  inner  germ-layer,  the  intes- 
tinal-glandular layer  (Fig.  92,  df),  remains  quite  unaltered, 
and  it  is  only  somewhat  later  that  it  begins  to  show  a  very 
uhallow,  channel-like  depression  along  the  central  line  of  the 
germ-shield,  immediately  below  the  notochord.  This  is  the 
intestinal  channel,  or  intestinal  furrow,  and  it  already  indi- 
cates the  future  destination  of  this  germ-layer.  For  as  the 
intestinal  channel  gradually  deepens,  and  its  lower  edges 
bend  towards  one  another,  it  assumes  the  form  of  a  closed 
tube,  the  intestinal  tube,  precisely  as  the  dorsal  furrow 
became  the  spinal  or  medullary  tube  (Fig.  92).  The  in- 
testinal-fibrous layer  (/),  which  lies  on  the  intestinal-glan- 
dular layer  (d),  naturally  follows  the  curve  of  the  latter. 
Thus  f -om  the  time  when  it  first  begins  to  develop,  the 
intestinal  wall  is  composed  of  two  strata,  internally  oi 
the  intestinal-glandular  layer,  externally  of  the  intestinal- 
fibrous  layer. 

The  formation  of  the  intestinal  tube~  is  so  far  similar  to 
that  of  the  spinal  tube,  that  in  both  cases  a  rectilineal  trench, 
or  furrow,  first  appears  along  the  central  line  of  a  flat  germ- 
layer.  The  edges  of  this  furrow  then  incline  towards  each 
other,  and  by  coalescence  form  a,  tube  (Fig.  93).  But  the 


FORMATION    OF   THE   INTESTINAL   TUBE. 


309 


two  processes  are  in  reality  quite  different.     For  the  spinal 
tube  closes  along  throughout  its  entire  length  into  a  cylin- 


FIG.  93. — Three  diagrammatic  transverse  sections  through  the  germ- 
shield  of  a  higher  Vertebrate,  showing  the  origin  of  the  tubular  rudimentary 
organs  from  the  bent  germ-layers.  In  Fig.  A  the  spinal  tube  (?i)  and  the 
intestinal  tube  (a)  are  still  open  trenches ;  the  primitive  kidneys  (M)  are  still 
simple  skin-glands.  In  Fig.  B  the  spinal  tube  (n)  and  the  dorsal  wall  have 
already  closed,  while  the  intestinal  tube  (a)  and  the  ventral  wall  are  still 
open  ;  the  primitive  kidneys  are  pinched  off.  In  Fig.  C  both  the  spinal  tube 
with  the  dorsal  wall  above,  and  the  intestinal  tube  with  the  ventral  wall 
below,  are  closed.  All  the  open  trenches  have  become  closed  tubes ;  the 
primitive  kidneys  have  penetrated  into  the  interior.  In  all  three  figures  the 
letters  indicate  the  same  parts:  h,  skin-sensory  layer ;  n,  spinal  tube,  or 
medullary  tube  ;  u,  primitive  kidneys ;  x,  notochord  ;  s,  vertebral  rudiments  ; 
r,  dorsal  wall;  b,  ventral  wall ;  c,  body-cavity  (cceZo-ma);  /,  intestinal-fibrous 
layer:  t,  primitive  artery  (aorta)  ;  v,  primitive  vein  (intestinal  vein);  d, 
intestinal-glandular  layer  ;  a,  intestinal  tube.  (Cf.  Plates  IV.  and  V.) 

drical  tube,  while  the  intestinal  tube  remains  open  in  the 
middle,  and,  till  a  much  later  stage,  this  cavity  remains 
connected  with  the  cavity  of  the  intestinal  germ-vesicle. 
The  connection  between  these  two  cavities  is  closed  only  at 
a  very  late  period,  by  the  formation  of  the  navel.  The 
closing  of  the  medullary  tube  proceeds  from  both  sides,  the 
right  and  left  edges  of  the  dorsal  furrow  coalescing.  The 


3IO  THE   EVOLUTION   OF   MAN. 

closing  of  the  intestinal  tube,  on  the  other  hand,  takes  place 
not  only  from  the  right  and  left,  but  by  a  concrescence  of  the 
walls  on  all  sides  of  the  intestinal  groove  towards  the  navel, 
as  a  central  point.  Moreover,  the  whole  process  of  the 
secondary  formation  of  the  intestine  in  the  three  higher 
classes  of  Vertebrates  is  most  closely  connected  with  the 
formation  of  the  navel,  with  the  "pinching  in"  of  the 
embryo  from  the  yelk-sac  (navel-vesicle).  (Cf.  Fig.  70,  p. 
283,  and  Plate  V.  Figs.  14  and  15.) 

In  order  to  be  quite  clear  about  these  points,  it  is  neces- 
sary to  bear  in  mind  the  relation  of  the  germ-shield  to  the 
germ-area  and  to  the  intestinal  germ-vesicle.  This  is  best 
accomplished  by  comparing  the  five  stages  which  are  repre- 
sented in  longitudinal  section  in  Fig.  94.  The  germ-shield  (e), 
which  at  first  protruded  only  slightly  from  the  surface  of 
the  germ-area,  soon  begins  to  raise  itself  from  the  latter,  and 
to  pinch  itself  off  the  intestinal  germ- vesicle.  During  this 
the  germ-shield,  seen  from  the  dorsal  side,  still  retains  its 
original  simple  sole-shape  (Figs.  86,  87,  p.  298).  There  is  as 
yet  no  appearance  of  any  distinction  into  head,  neck,  trunk, 
or  limbs.  But  the  germ-shield  has  grown  much  thicker,  espe- 
cially in  the  anterior  portion.  It  now,  therefore,  protrudes 
from  the  surface  of  the  germ-area  like  a  thick,  much  arched, 
oval  swelling,  and  begins  to  separate  and  free  itselJ 
completely  from  the  intestinal  germ-vesicle,  to  which  it  is 
attached  by  its  ventral  surface.  Ihe  progress  of  this  separ- 
ation renders  the  back  continually  more  curved;  in  proportion 
as  the  embryo  grows  and  becomes  larger,  the  germ-vesicle 
decreases  and  becomes  smaller,  till  at  last  it  hangs,  in  the 
form  of  a  small  bladder,  from  the  abdomen  of  the  embryo 
(Fig.  94,  5  ds).  In  consequence  of  the  processes  of  growth 


SEPARATION   OF   GERM-SHIELD.  31 1 

which  effect  this  separation,  a  furrow-like  depression  is  first 
formed  round  the  embryo-body  on  the  upper  surface  of  the 
germ- vesicle,  surrounding  it  like  a  trench ;  round  the  out^ 
side  of  this  trench  a  circular  wall,  or  dike,  is  formed  by 
the  elevation  of  the  adjoining  parts  of  the  germ- vesicle 
(Fig.  94,  afo). 

In  order  to  get  a  clear  and  connected  view  of  thia 
important  process,  we  may  compare  the  embryo  to  a 
fortress  surrounded  by  a  inoat  and  a  wall.  This  moat,  or 
trench,  consists  of  the  outer  part  of  the  germ-area,  and 
ceases  where  the  germ-area  passes  into  the  intestinal  germ- 
vesicle.  The  important  process  of  fission  in  the  middle 
germ-layer  which  occasions  the  formation  of  the  large 
body-cavity,  extends  over  the  whole  germ-area  along  the 
periphery  of  the  embryo.  At  first  the  extent  of  this 
middle  germ-layer  is  co-extensive  with  that  of  the  germ- 
area;  the  whole  remaining  part  of  the  intestinal  germ- 
vesicle  originally  consisting  only  of  the  two  original  germ- 
layers,  the  outer  and  the  inner.  Thus,  over  the  extent 
of  the  germ-area,'  the  middle  germ-layer  splits  into  the  two 
layers  which  we  knew  as  the  outer  skin-fibrous  layer, 
and  the  inner  intestinal-fibrous  layer.  These  two  layers 
separate  widely,  a  clear  fluid  collecting  between  them 
(Fig.  94, 3  am).  The  inner  layer,  the  intestinal-fibrous 
layer,  remains  lying  on  the  inner  layer  of  the  intestinal  germ- 
vesicle  (on  the  intestinal-glandular  layer).  The  outer  layer 
the  skin-fibrous  layer,  on  the  contrary,  attaches  itself  closety 
to  the  outer  layer  of  the  germ-area,  to  the  skin-sensory  layer, 
and  the  two  together  rise  up  from  the  intestinal  germ- vesicle. 
From  these  two  united  outer  layers,  a  connected  membrane 
now  arises.  Thia  is  the  circular  wall,  which  continues  to 


3I2 


THE   EVOLUTION    OF   MAN. 


FIG.  94. — Five  diagrammatic  longitudinal  sections  through  the  maturing 
mammalian  germ  and  its  egg-membranes.  In  Fig.  1-4,  the  longitudinal 
Bection  passes  through  the  sagittal  plane,  or  the  central  plane  of  the  body, 


THE  AMNION.  313 

;vhich  separates  the  right  and  loffc  halves  ;  in  Fig.  5,  the  germ  is  aoen  froTn 
'.he  left  side.  In  Fig.  1,  the  tufted  (d)  chorion  encloses  the  germ -vesicle, 
ihe  wall  of  which  consists  of  the  two  primary  germ-layers.  Between  the 
outer  (a)  and  inner  (i)  germ-layers,  the  middle  germ-layer  (m)  has  developed, 
cG-extensively  with  the  germ-area.  In  Fig.  2,  the  embryo  (e)  is  beginning  to 
separate  from  the  germ-vesicle  (ds),  while  the  wall  of  the  amnion-fold  la 
developing  round  it  (in  front  as  the  head-sheath,  frs,  in  rear  as  tail-sheath, 
ss).  In  Fig.  3,  the  edges  of  the  amnion-fold  (am)  meet  above  the  back  of 
the  embryo  and  thus  form  the  amnion-cavity  (ah) ;  while  the  embryo  (c) 
sepaiates  still  more  from  the  germ-vesicle,  the  intestinal  canal  (del)  is 
developed,  and  from  the  posterior  end  of  this,  the  allantois  (al)  grows  out. 
In  Fig.  4,  the  allantois  (al)  becomes  larger;  the  yelk-sac  (ds)  smaller.  In 
Fig.  5,  the  embryo  shows  the  gill-openings  and  the  rudiments  of  the  two 
pairs  of  limbs ;  the  chorion  has  formed  branching  tufts.  In  all  the  five 
figures  e  signifies  embryo ;  a,  outer  germ-layer ;  m,  middle  germ-layer ;  i, 
inner  germ-layer;  am,  amnion  (ks,  head-shenth;  ss,  tail- sheath) ;  ah, 
amnion-cavity;  as,  amnion-sheath  of  the  umbilical  cord;  Tch=ds,  intestinal 
germ-vesicle;  ds,  yelk-sac  (navel-vesicle)  ;  dg,  yelk-duct;  df,  intestinal- 
fibrous  layer;  dd,  intestinal-glandular  layer;  al,  allantois;  vl  =  hh,  region 
of  the  heart;  d,  yelk-membrane  (prochorion) ;  d',  tufts  on  prochorion;  sh, 
serous  membrane;  sz,  tufts  of  the  foregoing;  ch,  tufted  membrane  or 
chorion;  r,  the  space  between  the  amnion  and  chorion,  filled  with  fluid. 
(According  to  Kolliker.)  Compare  Table  V.  Fig.  14  and  15. 

raise  itself  higher  and  higher  around  the  entire  embryo,  and 
at  last  coalesces  above  it  (Fig.  94,  2,  3,  4,  5,  am).  To 
keep  up  the  simile  of  a  fortress  imagine  that  the  sur- 
rounding wall  of  the  fortress  becomes  extraordinarily  high, 
arid  towers  far  above  the  fortress.  Its  edges  arch  like 
the  crests  of  a  jutting  cliff  which  is  about  to  enclose  the 
fortress ;  they  form  a  deep  cavern,  and  at  last  grow  together 
above.  At  last  the  fortress  lies  entirely  within  the  cavern 
forme  by  the  concrescence  of  the  edges  of  this  mighty 
wall.  (Cf.  Figs.  95-98,  p.  319,  and  Plate  V.  Fig.  14.) 

These  two  outer  strata  of  the  germ-area,  rising  in  this 
way  in  the  form  of  folds  around  the  embryo  and  coalescing 
above  it,  at  last  form  a  spacious  sac-like  envelope  around 
it.  This  envelope  bears  the  name  of  germ-membrane, 


314  THE   EVOLUTION    OF   MAN. 

water-membrane,  or  amnion  (Fig.  94,  am).  The  embryo 
swims  in  a  watery  fluid,  which  fills  the  space  between  it 
and  the  amnion,  and  is  called  the  amnion-water,  or  germ- 
water  (Fig.  94,  4,  5  ak).  We  shall  return  hereafter  to  the 
significance  of  this  remarkable  formation.  It  is  of  no 
interest  to  us  at  present,  because  it  bears  no  direct  relation 
to  the  formation  of  the  body. 

Among  the  various  appendages,  the  significance  of  which 
we  shall  presently  recognize,  we  will  mention,  in  passing,  the 
allantois  and  the  yelk-sac.  The  allantois,  or  urinary  sac 
(Fig.  94,  3,  4  al),  is  a  pear-shaped  bladder,  which  grows  out 
from  the  hindmost  part  of  the  intestinal  canal:  the  inner- 
most portion  of  it  afterwards  changes  into  the  urinary 
bladder ;  the  outer  part,  with  its  vessels,  forms  the  founda- 
tion of  the  placenta.  In  front  of  the  allantois,  the  yelk-sac, 
or  navel  vesicle  (Fig.  94,  3,  4  ds),  the  remnant  of  the 
original  intestinal  germ- vesicle  (Fig.  94,  j  kti),  protrudes  from 
the  open  abdomen  of  the  embryo  (Fig.  94,  3,  4  ds).  In  a 
later  stage  of  development  of  the  embryo,  in  which  the  intes- 
tinal and  ventral  walls  are  nearly  closed,  this  hangs  out 
from  the  navel-opening  in  the  form  of  a  little  stalked 
bladder  (Fig.  94,  4,  5  ds).  Its  wall  consists  of  two  layers, 
the  inner  of  which  is  the  intestinal-glandular  layer,  the 
outer  the  intestinal-fibrous  layer.  It  is,  therefore,  a  direct 
continuation  of  the  intestinal  wall.  In  proportion  as  the 
embryo  grows  larger,  this  yelk -sac  becomes  smaller.  At 
first  the  embryo  looks  merely  like  a  small  appendage  on 
the  large  intestinal  germ-vesicle.  But,  on  the  contrary,  at 
a  later  period,  the  yelk-sac,  or  the  remnant  of  the  intestinal 
germ-vesicle,  looks  like  a  little  purse-shaped  appendage  of 
the  embryo  (Fig.  70).  Finally,  it  loses  all  importance.  The 


FORMATION  OF  THE  ALLANTOIS  AND  YELK-SAC.   31$ 

very  wide  opening  by  which  the  intestinal  cavity  at  first 
communicates  with  the  navel  bladder,  afterwards  grows 
continually  narrower,  and  at  last  altogether  disappears.  The 
navel,  the  little  pit-like  depression  which  appears  in  the 
middle  of  the  ventral  wall  of  the  developed  Man,  is  the 
p]aco  at  which  the  remains  of  the  germinal  vesicle,  the  navel 
bladder,  once  entered  the  intestinal  cavity,  and  by  which  it 
was  connected  with  the  intestine  in  the  course  of  its  evolu- 
tion. (Of.  Figs.  14  and  15  on  Plate  V.) 

The  formation  of  the  navel  takes  place  at  the  same  time 
as  the  closing  of  the  outer  ventral  wall.  The  ventral  wall 
originates  in  exactly  the  same  way  as  the  dorsal  wall ; 
both  are  formed  essentially  from  the  skin-fibrous  layer, 
and  are  covered  outwardly  by  the  horn-plate,  the  peripheric 
part  of  the  skin-sensory  layer.  Both  are  formed  by  the 
modification  of  the  animal  germ-layer  into  a  double  tube; 
above,  at  the  back,  the  vertebral  canal,  which  encloses  the 
spinal  tube, — below,  at  the  abdomen,  the  wall  of  the  body- 
cavity,  which  encloses  the  intestinal  tube  (Fig.  93,  p.  309). 

We  will  first  notice  the  formation  of  the  dorsal  wall, 
and  then  that  of  the  ventral  wall  (Figs.  95-98).  In  the 
centre  of  the  dorsal  surface  of  the  embryo  the  spinal  tube 
(mr)  lies,  originally  immediately  below  the  horn-plate  (1i), 
from  the  central  part  of  which  it  has  separated.  But,  at 
a  later  period,  the  primitive  vertebral  plates  (uw)  grow 
from  the  right  and  the  left  so  as  to  penetrate  between 
these  two  originally  connected  parts  (Figs.  97,  98).  The 
upper  inner  edges  of  the  two  primitive  vertebral  plates 
wedge  themselves  in  between  the  horn-plate  and  the  spinal 
tube,  press  these  two  apart,  and  finally  coalesce  between 
them  in  a,  suture  corresponding  with  the  central  line  of 
23 


3l6  THE  EVOLUTION   OF  MAN. 

the  back.  The  closing  is  effected  in  exactly  the  same  way 
as  that  of  the  spinal  tube,  which  is  now  entirely  enclosed 
by  the  vertebral  canal.  In  this  way  the  dorsal  wall  i? 
formed,  and  the  spinal  tube  lies  quite  in  the  interior  (Fig. 
98).  In  the  same  way  the  primitive  vertebral  mass  grows 
lower  down  round  the  notochord  (chorda  dorsalis),  there 
forming  the  vertebral  column.  In  this  lower  part  the  inner 
under  edge  of  the  primitive  vertebral  plates  on  each  side 
splits  into  two  laminae,  the  upper  of  which  passes  in 
between  the  notochord  and  the  spinal  tube,  while  the  under, 
on  the  contrary,  penetrates  between  the  notochord  and  the 
intestinal  tube.  These  two  laminae,  by  meeting  from  eacli 
side  above  and  below  the  notochord,  completely  enclose  the 
latter,  and  thus  form  the  tubular  outer  notochord-sheath, 
the  skeleton-forming  layer,  from  which  the  vertebra] 
column  arises  (Figs.  97,  98).  (Of.  Figs.  3-6  on  Plate  IV., 
and  the  following  chapter.) 

Processes  similar  to  these  which  take  place  above,  on 
the  back,  during  the  formation  of  the  dorsal  wall,  are 
observed  below,  on  the  abdomen,  during  the  formation  of 
the  ventral  wall  (Fig.  98,  67t).  Here  the  side-plates  grow 
together  round  the  intestine  in  a  similar  way  to  that  in 
which  the  intestine  itself  closed.  The  outer  part  of  the 
side-plates  forms  the  ventral  wall,  or  the  lower  body- wall, 
while  on  the  inner  side  of  the  amnion-fold,  which  has  been 
mentioned,  the  two  side-plates  curve  more  and  grow  toward 
each  other  from  right  and  left.  While  the  intestinal  canal 
is  closing,  the  closing  of  the  ventral  wall  is  also  taking  place 
from  all  sides.  Thus  the  ventral  wall,  which  encloses  the 
whole  ventral  cavity  below,  also  originates  from  two  halves, 
from  the  two  side-plates,  which  incline  toward  each  other ; 


DEVELOPMENT    OF   THE    DORSAL   AND   VENTRAL   WALLS.    317 

these  grow  toward  each  other  from  all  sides,  and  at  last 
unite  in  the  navel  at  the  centre.  We  must,  therefore,  dis- 
tinguish between  two  navels,  an  inner  and  an  outer.  The 
inner  or  intestinal  navel  is  the  point  at  which  the  in- 
testinal wall  finally  closes,  at  which  the  communication 
between  the  intestinal  cavity  and  the  cavity  of  the  yelk- 
sac  was  cut  off  (Fig.  70).  The  outer  or  skin-navel  is  the 
point  at  which  the  ventral  wall  finally  closes,  and  which 
even  in  adults  is  visible  as  a  depression.  In  each  concrescence 
two  secondary  germ-layers  are  concerned ;  at  that  of  the 
intestinal  wall,  the  intestinal-glandular  layer  and  the  in- 
testinal-fibrous layer ;  at  that  of  the  ventral  wall,  the  skin- 
fibrous  layer  and  the  skin-sensory  layer.  The  intestinal 
wall,  as  a  whole,  arises,  therefore,  from  the  entoderm,  and 
the  ventral  wall  (and,  indeed,  the  entire  body-wall)  from 
the  exoderm.95 

The  processes  by  which  the  double  tubular  rudiment  of 
the  body  originates  from  the  four-layered  germ-disc  are, 
therefore,  really  very  simple.  They  are  not,  however,  at 
once  easily  understood,  nor  is  it  easy  to  describe  them. 
Very  much,  doubtless,  yet  remains  obscure  to  the  reader, 


THE   EVOLUTION   OF  MAN. 


Fig.  96. 


EXPLANATION   OF   FIGURES. 


Fig.  98. 

FIGS.  95-98.— Transverse  sections  through  embryo  Chick:  Fig.  95,  the 
second  day  of  incubation ;  Fig.  96,  the  third ;  Fig.  97,  the  fourth  ,  and  Fig. 
98,  the  fifth.  Figures  95-97  are  after  Kolliker  (magnified  about  100  times) ; 
Fig.  98,  after  Kemak  (magnified  about  20  times). 

h,  horn-plate ;  Vnr,  spinal  tube ;  ung,  primitive  kidney  duct ;  wn,  pri- 
mitive kidney  vesicle ;  hp,  skin-fibrous  layer ;  m  =  mu  =  mp,  muscle- 
plate  ;  ww,  primitive  vertebral  plate  (ivh,  membranous  formation  of  the 
vertebral  body ;  wb,  of  the  vertebral  arch ;  wq,  of  the  rib,  or  transverse 
apophysis)  ;  uwh,  primitive  vertebral  cavity  ;  ch,  spinal  axis,  or  aoto- 
chord ;  sh,  notochord-sheath  ;  bh,  ventral  wall;  g,  posterior;  v,  anterior 
nerve  roots  of  the  spinal  marrow ;  a  =  of  =  am,  amnion-fold ;  p,  body- 
cavity,  or  ccelom ;  df,  intestinal-fibrous  layer ;  ao,  primitive  aortas ;  sa, 
secondary  aorta ;  vc,  principal  reins ;  d  —  dd,  intestinal-glandular  layer ; 
dr,  intestinal  groove.  In  Fig.  95,  the  greater  part  of  the  right  half  of  the 
cross-section  is  omitted,  and  in  Fig.  96  the  greater  part  of  the  left  half. 
Only  a  small  part  of  the  wall  of  the  yelk-sac,  the  remnant  of  the  germ- 
vesicle,  which  lies  below,  is  shown. 

especially  to   those  who  are  not  at  all  familiar  with  the 


32O  THE   EVOLUTION   OF  MAN 

anatomical  features.  If,  however,  the  subsequent  stages  of 
development,  which  throw  light  on  their  predecessors, 
are  accurately  noted,  and  especially,  if  the  transverse 
sections  in  the  preceding  figures  and  in  Plate  IV.,  repre- 
senting the  complete  vertebrate  body  and  its  germ,  are 
carefully  compared,  the  reader  will  probably  obtain  a  clear 
conception  of  the  main  features  of  mammalian  Ontogeny. 
A  close  and  thoughtful  comparison  of  the  transverse  sections 
is  of  the  greatest  importance  in  this  respect. 

It  is  true,  however,  that  a  deeper,  phylogenetic  know- 
ledge of  these  complex  processes  can  only  be  gained  with 
the  aid  of  Comparative  Anatomy  and  Ontogeny.  These 
teach  us  that  the  ontogenetic  process  which  we  have 
described  as  resulting  in  the  formation  of  the  Vertebrate 
must  be  explained  as  kenogenetic,  and  that,  in  consequence 
of  continual  embryonic  adaptation,  these  processes  have 
departed  very  widely  from  the  original  palingenetic  form. 
The  Amphioxus  alone  of  all  living  Vertebrates  has,  in  con- 
sequence of  tenacious  heredity,  approximately  retained  the 
palingenetic  form.96  (C£  Chapters  XIII.  and  XIV.) 

As  yet  we  have  paid  no  attention  to  the  various  sections 
which  are  distinguishable  in  the  length  of  the  body :  the 
head,  neck,  breast,  abdomen,  tail,  etc.  The  transverse 
sections  do  not  help  us  in  this  respect,  and  we  must,  there- 
fore, closely  observe  the  articulation  in  the  longitudinal  axis 
of  the  mammalian  body. 


HABCKEL'S  EVOLUTION  OF  MAN.  PLATE  IV. 

TRANSVERSE  SECTIONS. 


IAECKEL'S  EVOLUTION  OF  MAN.  PLATE  V 

LONGITUDINAL  SECTIONS. 


EXPLANATION  OF  PLATES  IV.  AND  V. 

The  two  Plates  IV.  and  V.  exhibit,  partly  ontogenetically  and  partly 
pbylogenetically,  the  mode  in  which  the  human  body  arises  from  the  germ- 
layers.  Plate  IV.  contains  only  diagrammatic  transverse  sections  (through 
the  sagittal  and  transverse  axes)  ;  Plate  V.  contains  only  diagrammatic  longi- 
tudinal sections  (through  the  sagittal  and  longitudinal  axes),  seen  from  the 
left  side.  The  four  secondary  germ-layers  and  their  products  are  distin- 
guished throughout  by  the  same  four  colours,  namely :  (1)  the  skin-sensory 
layer  is  orange ;  (2)  the  skin-fibrous  layer,  blue ;  (3)  the  intestinal-fibrous 
layer,  red ;  and  (4)  the  intestinal-glandular  layer,  green.  In  all,  the  letters 
indicate  the  same  parts.  In  Fig.  1  and  9  alone  the  two  primary  germ- 
layers  are  represented — the  outer,  or  skin-layer,  orange ;  the  inner,  or 
intestinal  layer,  green.  In  all  the  figures  the  dorsal  surface  of  the  body  is 
uppermost,  the  ventral  surface  underneath.  All  organs  proceeding  from 
the  skin-layer  are  marked  with  blue  letters ;  all  those  proceeding  from  the 
intestinal  layer,  with  red  letters.97 

PLATE  IV. — DIAGRAMMATIC  TRANSVERSE  SECTIONS. 

PIG.  1. — Transverse  section  through  the  Gastrula.  (Compare  Fig.  9, 
longitudinal  section,  and  Figs.  22-28,  p.  193.)  The  whole  body  is  formed  by 
the  intestinal  tube  (d)  ;  the  wall  of  this  consists  solely  of  the  two  primary 
germ-layers. 

FIG.  2. — Transverse  section  through  the  larva  of  the  Amphioxus,  in  tha 
early  stage  in  which  the  body  consists  merely  of  the  four  secondary  germ- 
layers.  The  intestinal  tube  (d),  foi-med  of  the  intestinal  layer,  is  separated 
fiom  the  body-wall  by  the  coelom  (c),  which  is  formed  of  the  skin-layer. 

FIG.  3. — Transverse  section  through  the  germ-disc  of  a  higher  Vertebrate, 
with  the  rudiments  of  the  earliest  organs.  (Compare  the  transverse  section 
of  the  embryo  Chick  at  the  second  day  of  incubation,  Fig.  92.)  The  spinal 
tube  (m)  and  the  primitive  kidneys  (M)  are  separated  from  the  horn-plate  (h). 
On  both  sides  of  the  notochord  (cfc)  the  primitive  vertebrae  (uw)  and  the 
side-layers  are  difforeutiated.  Between  the  skin-fibrous  layer  and  the  iiites- 


322  THE   EVOLUTION   OF   MAN. 

tinal-fibrons  layer,  the  first  rudiment  of  the  body-cavity,  or  the  coelom  (c),  is 
visible  ;  under  it  are  the  two  primitive  aortas  (t). 

FIG.  4. — Transverse  section  through  the  germ-disc  of  a  higher  Vertebrate, 
somewhat  farther  developed  than  in  Fig.  3.  (Compare  the  transverse 
section  of  the  embryo  Chick  at  the  third  day  of  incubation,  Fig.  95  and  96, 
p.  317.)  The  spinal  tube  (m)  and  the  notochord  (c/i)  are  already  beginning 
to  be  enclosed  by  the  primitive  vertebrae  (uw),  in  which  the  muscle-plates, 
bone-plates,  and  nerve-roots  are  becoming  distinct.  The  primitive  kidneys 
(«)  are  already  completely  separated  from  the  horn-plate  (h)  by  the  leather- 
plate  (1)  ;  c,  the  coelom;  t,  the  aortas.  The  skin-layer,  rising  around 
the  embryo,  forms  the  amnion-fold  (am) ;  this  gives  rise  to  a  hollow  space 
(0)  between  the  amnion-fold  and  the  wall  of  the  yelk-sac  (<£s). 

FIG.  5. —Transverse  section  through  the  pelvic  region  and  the  posterior 
limbs  of  the  embryo  of  a  higher  Vertebrate.  (Compare  the  transverse 
section  through  Chick  at  the  fifth  day  of  incubation,  Fig.  120.)  The  spinal 
tube  (m)  is  already  entirely  enclosed  by  the  two  curving  halves  of  the 
vertebras  (wl),  and  similarly  the  notochord  and  its  sheath  by  the  two  halves 
of  the  vertebral  body  (wk).  The  leather-plate  (I)  has  entirely  separated 
from  the  muscle-plate  (mp).  The  horn-plate  (h)  has  thickened  very  much 
at  the  head  of  the  posterior  limbs  (x).  The  primitive  kidneys  (w)  are  pro- 
minent in  the  coelom  (c),  and  lie  very  near  the  germ-epithelium,  or  the 
rudimentary  sexual  glands  (&)•  The  intestinal  tube  (d)  is  attached  to  the 
dorsal  surface  of  the  body  by  the  mesentery  (g),  beneath  the  main  artery  (t), 
and  the  two  principal  veins  (77).  Below,  in  the  centre  of  the  ventral  wall, 
the  stalk  of  the  allantois  (al)  is  visible. 

FIG.  6. — Transverse  section  through  a  developed  Primitive  Fish,  or  some 
other  Vertebrate  of  a  low  order.  The  parts,  on  the  whole,  bear  the  same 
relation  to  each  other  as  in  the  preceding  transverse  section,  Fig.  5,  and  are 
marked  in  the  same  way.  But  the  sexual  glands  (k)  have  developed  into 
ovaries,  and  the  primitive  kidneys  are  transferred  into  oviducts,  which  open 
into  the  coalom.  The  two  side  protuberances  (lb)  of  the  intestinal  tube  (d) 
indicate  the  intestinal  glands,  for  example,  the  liver.  Below  the  intestinal 
tube,  in  the  intestinal  wall,  lies  the  intestinal  vein  (v) ;  above  the  intestinal 
tube  lies  the  aorta  (t),  and  above  this,  again,  the  two  principal  veins  (n). 

FIG.  7. — Transverse  section  through  one  of  the  higher  Worms  (through 
the  head  of  an  Annelid),  showing  its  essential  agreement  with  the  Verte- 
brates in  the  construction  of  the  body  from  the  four  secondary  germ, 
layers.  It  should  be  carefully  compared  with  the  diagrammatic  trans- 
verse section  through  the  low  Vertebrate,  Fig.  6  :  m,  the  "  brain,"  or  "upper 
throat  ganglion."  The  leather-plate  (V)  and  the  muscle-plate,  which  lies  below 
the  former,  have  differentiated  from  the  skin-fibrous  layer.  The  muscle-layer 
has  separated  into  an  outer  circular  muscle-stratum  and  a  long  inner  stratum, 
and  the  muscle  of  the  latter  has  distributed  itself  into  dorsal  muscles  (r)  and 
voutial  muscles  (b).  The  two  are  separated  by  the  pi-iuiitive  kidneys  (u), 


EXPLANATION    OF    PLATE   V.  323 

erhich  extend  from  the  horn-plate  (ft)  to  the  coelom  (c).  Here  tLe  primitive 
kidneys  have  a  funnel-shaped  opening,  through  which  they  carry  ont  the 
ovules,  which  fall  from  the  ovaries  (fc)  into  the  coelom.  The  intestinal  tube 
(d)  has  glands  on  its  surface  (liver-vesicles,  Z6).  Below  it  lies  the  ventral  vessel 
(the  intestinal  vein,  i>),  above  it  the  dorsal  vessel  (the  aorta,  t).  The  position 
and  origin  of  all  these  primitive  organs  is  entirely  the  same  in  Man  and  every 
other  Vertebrate,  as  in  the  Worms.  The  only  essential  difference  is  that  in 
the  Vertebrates  a  notochord  is  developed  between  the  spinal  tube  and  the 
intestinal  tube. 

FIG.  8. — Transverse  section  through  the  human  thorax.  The  epinal  tube 
(m)  is  entirely  enclosed  by  the  developed  circular  vertebrae  (w).  A  curved 
rib  proceeds  right  and  left  from  the  vertebra,  supporting  the  wall  of  the 
breast  (rp).  Below,  on  the  ventral  surface,  between  the  right  and  left  rib, 
lies  the  breast-bone,  or  sternum  (6b).  Without,  above  the  ribs,  and  the 
muscles  between  the  ribs,  lies  the  outer  skin,  formed  from  the  leather-plate 
(I)  and  the  horn-plate  (ft,).  The  greater  part  of  the  breast-cavity  (or  the 
anterior  part  of  the  coelom,  c)  is  occupied  by  the  two  lungs  (lu),  in  which 
the  branches  of  the  trachea  ramify  like  a  tree.  These  all  open  together 
into  the  unequal  branches  of  the  trachea  (Zr),  which  opens  further  up 
at  the  neck  into  the  oesophagus  (sr).  Between  the  intestinal  tube  and  the 
vertebral  column,  lies  the  aorta  (t).  Between  the  trachea  and  the  sternum 
lies  the  heart  divided  by  a  partition  wall  into  two  halves.  The  left  heart 
(hi)  contains  only  arterial,  the  right  (hr)  only  venous  blood.  Each  half  of 
the  heart  is  divided  by  a  valved  opening  into  an  auricle  and  a  ventricle. 
The  heart  is  here  represented  diagrammatically  in  its  (phylogenetic)  original 
symmetrical  position  (in  the  centre  of  the  ventral  side).  In  the  developed 
human  being,  and  in  apes,  the  heart  lies  in  an  unsymmetrical  and  oblique 
position,  inclined  to  the  left. 

PLATE  V.— DIAGRAMMATIC  LONGITUDINAL  SECTIONS. 

FIG.  9. — Longitudinal  section  through  a  Gastrula.  (Compare  Fig.  1, 
transverse  section.)  The  intestinal  cavity  (J)  opens  in  front  through  the 
mouth  (o) .  The  body  consists  merely  of  the  two  primary  germ-layers. 

FIG.  10. — Longitudinal  section  through  an  hypothetical  Primitive  Worm 
(Prolhelmis) ,  the  entire  body  of  which  consists  of  the  four  secondary  germ- 
layers.  The  intestinal  tube  (d)  is  still  very  simple;  but  the  anterior  and 
posterior  intestines  begin  to  grow  distinct.  The  mouth  (o)  is  still  the  anus 
also. 

FIG.  11. — Longitudinal  section  through  a  low  Coelomate  Worm.  The  primi- 
tive brain  (m),  or  the  first  nerve-centre  overlying  the  throat,  has  separated 
from  the  horn-plate  (h).  The  intestinal  tube  has  acquired  a  second  posterior 
anal  opening  (a)  in  addition  to  the  mouth-opening  (a)  in  front.  A  skin- 
gland  has  developed  into  primitive  kidneys  (M)  and  opens  into  the  body 


324  THE   EVOLUTION   OF   MAX. 

cavity  (c),  which  has  formed  between  the  skin  •fibrous  layer  and  the  intes 
tinal-fibrous  layer. 

FIG.  12. — Longitudinal  section  through  an  hypothetical  Worm  (Chordo- 
nium),  which  was  among  the  common  parent-forms  of  Vertebrates  and 
Ascidians.  The  primitive  brain  (m)  has  lengthened  into  an  elongated  spinal 
tube.  Between  this  spinal  tube  and  the  intestinal  tube  (d),  the  notochord 
(ch)  has  developed.  The  intestinal  tube  has  differentiated  into  two  divisions, 
an  anterior  gill-intestine  (with  three  pairs  of  gill-openings,  fcs)  which 
nerves  for  breathing,  and  a  posterior  stomach-intestine  (with  a  liver- 
appendage,  Zb)  which  serves  for  digestion.  In  front,  at  the  head-extremity, 
an  organ  of  sense  (q)  has  developed.  The  primitive  kidney  (w)  opens  into 
the  body-cavity  (c). 

FIG.  13. — Longitudinal' section  through  a  Primitive  Fish  (Proselachius) , 
closely  related  to  the  existing  Sharks,  and  hypothetical  ancestors  of  Man 
(the  fins  are  omitted).  The  spinal  tube  has  differentiated  into  the  five 
primitive  brain-bladders  (m^ — ms)  and  the  spinal  marrow  (we).  (Compare 
Figs.  15  and  16.)  The  brain  is  enclosed  in  the  skull  (s),  the  spinal  marrowin 
the  vertebral  canal  (above  the  spinal  marrow,  the  vertebi'al  arches  (ivb) ; 
under  it  the  vertebral  bodies  (wk)  ;  under  the  latter  the  origin  of  the  ribs  is 
indicated).  In  front  an  organ  of  sense  (q,  nose  or  eye)  has  developed  from 
the  horn-layer, — at  the  back,  the  primitive  kidney  («).  The  intestinal  tube 
(d)  has  differentiated  into  the  following  parts,  lying  one  behind  another : 
the  mouth-cavity  (mh),  the  throat-cavity  with  six  pairs  of  gill-openings 
(ks),  the  swimming-bladder  (= lungs,  lu),  the  oesophagus  (sr),  the  stomach 
(mg),  the  liver  (Ib)  with  the  gall-bladder  (i).  the  small  intestine  (dd),  and 
the  rectum  with  the  anus  (a).  Below  the  throat-cav.'ty  lies  the  heart,  with 
the  auricle  (hv")  and  the  ventricle  (hk). 

FIG.  14. — Longitudinal  section  through  a  human  embryo  of  three  weeks, 
showing  tne  relation  of  the  intestinal  tube  to  its  appendages.  In  the  centre 
the  long-stalked  yelk-sac  (or  the  navel-vesicle,  ds)  projects  from  the  intes- 
tinal tube  (ds) ;  similarly  the  long-stalked  allantois  (al)  projects  from  the 
intestine  at  the  back.  The  heart  (hz)  is  visible  beneath  the  anterior  intes- 
tine. Amnion-cavity  (ah). 

FIG.  15. — Longitudinal  section  through  a  human  embryo  of  five  weeks. 
(Compare  Fig.  14.)  The  amnion  and  the  placenta,  with  the  urachus,  are 
omitted.  The  spinal  tube  has  differentiated  into  the  five  primitive  brain-blad- 
ders (wi1-m5),  and  the  spinal  marrow  (rrea).  (Compare  Figs.  13  and  16.)  The 
skull  (s)  is  formed  around  the  brain  ;  below  the  spinal  marrow  the  series  of 
vertebral  bodies  (wk).  The  intestinal  tube  has  differentiated  into  the 
following  divisions,  lying  one  behind  another  :  the  throat-cavity  with  three 
pairs  of  gill-openings  (its),  the  lung  (lu),  the  oesophagus  (sr),  the  stomach 
(mg),  the  lirer  (IV),  the  coil  of  the  small  intestine  (dd),  into  which  the 
)  elk-sac  (ds)  opens,  the  urinary  bladder  (Tib),  and  the  rectum.  Heart  (hz). 

FIG.  16  — Longitudinal  section  through  developed  human  female.     All 


EXPLANATION    OF   PLATE   V.  325 

tlie  parts  are  perfectly  developed,  but  diagramtnatically  reduced  and  sim- 
plified, in  order  to  exhibit  clearly  their  relative  positions  and  their  relations 
to  the  four  secondary  germ-layers.  In  the  brain,  the  five  original  bra'iu- 
bladders  (Fig.  15,  m^-ms)  have  been  differentiated  and  transformed  in  the 
manner  peculiar  to  the  higher  mammals :  ma,  fore  brain  (cerebrum),  out- 
weighing and  covering  all  the  other  four  brain  bladders;  ma,  twixt  brain 
("  the  centre  of  sight  ") ;  m3,  mid  brain  ("  the  four  bulbs  ")  ;  m4,  hind 
brain  (cerebellum)  ;  ms,  after  brain,  or  prolonged  marrow  (medulla 
oblongata),  passing  into  the  spinal  marrow  (me).  The  brain  is  enclosed  in 
the  skull  (s),  the  spinal  marrow  by  the  vertebral  canal:  above  the  spinal 
marrow  the  vertebral  arches  and  spinal  processes,  under  it  the  vertebral 
bodies  (wk).  The  intestinal  tube  has  differentiated  into  the  following  parts 
lying  one  behind  another :  the  mouth-cavity,  the  throat-cavity  (in  which  at 
an  earlier  period  the  gill-openings,  ks,  were  situated),  the  trachea  (Ir)  with 
the  lungs  (lu),  the  oesophagus  (sr),  the  stomach  (mg)>  the  liver  (lb),  with 
the  gall-bladder  (t),  the  ventral  salivary  gland,  or  pancreas  (p),  the  small 
intestine  (dd),  the  large  intestine  (dc),  and  the  rectum  with  the  anus  (a). 
The  body-cavity,  or  coalom  (c),  is  divided  by  the  diaphragm  (z)  into  two 
distinct  cavities ;  the  breast-cavity  (c),  in  which  the  heart  (hz)  lies  in  front  of 
the  lungs,  and  the  ventral  cavity  in  which  most  of  the  intestines  lie.  In  front 
of  the  rectum  lies  the  sheath  (vagina,  vg),  which  leads  into  the  uterus  (/)  ; 
in  this  the  embryo,  indicated  here  by  a  small  germ-membrane  vesicle  (e),  is 
developed.  Between  the  uterus  and  the  os  pubis  lies  the  vesica  urince  (hb), 
the  remains  of  the  stalk  of  the  allantois.  The  horn-plate  (/i)  as  the  outer 
skin,  covers  the  whole  body,  and  also  forms  the  coating  of  the  cavities  of 
the  month,  the  anus,  the  vagina,  and  the  uterus.  The  milk  glaudfl,  or  nuoan".\vt 
find),  ;uv  also  origiually  foTmud  L'oru  the  horu-pbifco. 


326 


TUB    EVOLUTION    OF   MAN. 


ALPHABETICAL   LIST 
Of  the  Meaning  of  the  Letters  in  Plates  IV.  and  V. 

N.B.— The  skin-sensory  layer  is  indicated  by  orange,  the  skin-fibrorb 
layer  by  blue,  the  intestinal-fibrons  layer  by  red,  and  the  intestinal-glaudulm 
layer  by  green. 


a,     anal  opening 

m  —  ms,  the  five  brain-bladilcra 

ah,  amnion-cavity 

m  ,  fore-brain 

al,    allantois  (urine  sao) 

m  ,  twixt-brain 

am,  amnion 

m  ,  mid-brain 

b,      ventral  muscles 

m  t  hind-brain 

bb,    breast-bone  (sternum) 

m  ,  after-brain 

c,      body-cavity  (cceloma) 

m  ,  spinal  cord  (medulla  spinalis) 

c,,     breast-cavity  (cavitas  pleurae) 

md,  milk-glands  (mammas) 

c,,,    ventral  cavity  (cavitas  peritonei) 

mg,  stomach 

ch,    notochord  (chorda) 

mh,  month  -cavity 

d,      intestinal  tube  (tractu*) 

mp,  muscle-plate  (muscularis) 

dc,    large  intestine  (colon) 

n,     principal  veins 

dd,    small  intestine  (ileum) 

o,     month-opening  (osculum) 

ds,    yelk-sac  (navel-vesicle) 

p,     ventral  salivary  gland  (pancreas) 

e,      embryo  or  germ 

q,     organ  of  sense 

f,       matrix  (uterus) 

r,      muscles  of  the  back 

g,      mesentery  (mesenterium) 

rp,    ribs  (costce) 

h,     horn-plate  (ceratina) 

s,      skull  (cranium) 

hb,    urinary  vesicle  (vesica  urines) 

sb,    os  pubis 

hie,    ventricle  of  heart 

ah,    throat-cavity  (pharynx) 

hi,    left  (arterial)  heart 

tr,    gullet  (oesophagus) 

hr,    right  (venons)  heart 

t,      aorta  (main  artery) 

hv,    auricle  (atrium) 

«,      primitive  kidney  (proton  ephron) 

hz,    heart  (cor) 

uw,  embryonic  vertebra  (metameron) 

i,       gall-bladder  (vesica  fellea) 

v,      intestinal  vein  (primitive  vein) 

k,      germ-glands  (sexual  glands) 

vg,    vagina 

ks,     gill-openings  (throat-openings) 

to,     vertebra 

I,      leather-plate  (corium) 

wb,  vertebral  arches 

Ib,     liver  (hepar) 

wk,    vertebral  bodies 

If,    windpipe  (trachea) 

x,       legs,  or  limbs 

lu,    lung  (pulmo) 

y,       space  between  the  amnion  and 

>n,     medullary  tube  (tubus    medul- 

the  yelk-sac 

larie) 

x,       midriff  (diaphragma) 

(    327    ) 


TABLE   VII. 

Systematic  Survey  of  the  Development  of  the  Organic  Systems  of  Man  from 
the  Germ-layers.     (Cf.  Plates  IV.  and  V.) 


J 

!1.  Outer-skin  (epidermis). 

I. 

2.  Appendages  of   the    epi- 

a. 

First 
secondary 
germ-layer. 

Horn-plate. 
Lamella  ceratina. 

dermis  (hair,  nails,  etc.) 
3.  Glands  of  the    epidermit 
(perspiratory,  sebaceous, 
lacteal  glands). 

Skin-sensory 

n. 

4.  Spinal  marrow)  medullary 

A. 
Outer  Primary 

layer. 
(Skin-stratum, 
'       Baer.) 

Mar  row  -plate. 
Lamella  medullarit. 

5.  Brain                J     tube. 
6.  Organs  of  the  senses  (es- 
sential part). 

Germ-layer. 

Lamina 

III. 

!1.  Primitive  kidneys  (?)  and 

Skin-layer. 

neurodermalis, 

a. 

Primitive  kidney 

the  outlets,  which  arise 
from  them  for  the  sexual 

(Animal 
Germ-layer, 
Baer.) 

plate. 
Lamella  renalis. 

products  (perhaps  from 
the  skin-fibrous  layer?  ?) 

Exoderma. 
Isimina 
dermalis,  II. 

6. 

Second 
secondary 

IV. 

Leather-plate, 
Lamella  coriaria. 

!8.  True  skin   (coriumi  and 
skin-muscle  stratum  ? 

germ-layer. 

9.  Trunk  -  muscle    stratum 

Skin-fibrous 

(side    muscles    of    the 

layer        ' 
(Flesh-stratum, 
Baer.) 

V. 

Flesh  -plate. 
Lamella  carnosa. 

trunk,  etc.). 
10.  Inner  skeleton  (chord,  ver- 
tebral column,  etc.). 
11.  Exocoelar?  (parietal  cuelom- 

Lamina 

epithelium). 

inodi:rmalis,  H. 

12.  Male  germ-epitheliiim  (ru- 

dimentary testes)  ?  't 

Body-Cavity  (CteZoma):  A  space  between  the  skin-layer  i 
the  body-wall  and  the  intestinal  wall,  tilled  with  lyi 


nd  the  intestinal  layer,  between 
iph  (colourless  blood). 


/ 

' 

11  Female     germ-epithelium 

1             c 

(rudimentary  ovary)  .'  ? 

c. 

H.  Endocoelar?  (visceral  cce- 

Third 

VI. 

lum-epithelium). 

• 

secondary 
germ-layer. 

Intestinal-   , 
fibrous  layer. 

Vascular  plate. 
Lamella  vasculosa. 

15.  Main  blood-vessels  (heart, 
primitive  arteries,  prim- 
itive veins). 
16.  Blood-vessel  glands  (lym- 
phatic      glands       and 

Inner 

Primary 

(Vascular-stra- 

spleen). 

Germ-layer. 

Intestinal 
layer. 
(Vegetative 

Lamina  ino- 
gastralis,  H. 

Vii.                  fl7.  Mesentery  (mesenterium). 
...  ..           .      '      .    .        J  18.  Intestinal  -  muscle      wall 
Mppentery-plate.   <          (and  fibrous  intestinal 
Lamella  mesenterica.     ^           membranes). 

Germ-layer, 

d. 

Baer.) 

Fourth 

Entoderma. 
Lamina 

secondary 
germ-layer. 

19.  Intestinal  epithelium.  (.In- 
ner cell-coating  of  the 

gcutralis,  H. 

Intestinal- 

vm. 

intestinal  tube.) 

glandular 
layer. 

Mucous  plate. 
Lamella  mucosa. 

30.  Intestinal    gland    epithe- 
lium.   (Inner  cell-coat- 
ing    of    the    intestinal 

(Mucous-stra- 

\         glands.) 

tum,  Baer.) 

Lamina  myco~ 

\    gastralis,  11. 

CHAPTER  XL 

GENERAL  STRUCTURE  AND  ARTICULATION  OF  THK 
INDIVIDUAL. 

Essential  Agreement  between  the  Chief  Palingenetic  Germ  Processes  in  the 
case  of  Man  and  in  that  of  other  Vertebrates. — The  Human  Body,  like 
that  of  all  Higher  Animals,  develops  from  Two  Primary  and  Four 
Secondary  Germ-layers. — The  Skin-sensory  Layer  forms  the  Horn-plate, 
the  Medullary  Tube,  and  the  Primitive  Kidneys.— The  Middle  Layer 
(Me  oderrn)  breaks  up  into  the  Central  Notochord,  the  Two  Primitive 
Vertebral  Cords,  and  the  Two  Side-layers.— The  latter  split  up  into  the 
Skin-fibrous  Layer  and  the  Intestinal-fibrous  Lnyer. — The  Intestinal- 
glandular  Layer  forms  the  Epithelium  of  the  Intestinal  Canal,  and  of 
all  its  Appendages. — Ontogenetic  and  Phylogenetic  Fission  of  the 
Germ-layers. — Formation  of  the  Intestinal  Canal. — The  Two-layered 
Globular  Intestinal  Germ-vesicle  of  Mammals  represents  the  Primitive 
Intestine. — Head  Intestinal  Cavity,  and  Pelvic  Intestinal  Cavity. — 
Mouth  Groove  and  Anal  Groove. — Secondary  Formation  of  Mouth 
and  Anus. — Intestinal  Navel  and  Skin-navel. — Movement  of  the  Primi- 
tive Kidneys  from  the  Outside  to  the  Inside. — Separation  of  the 
Brain  and  Spinal  Marrow. — Rudiments  of  the  Brain-bladders. — The 
Articulation  or  Metameric  Structure  of  the  Body. — The  Primitive 
Vertebrae  (Trunk- Segments,  or  Metamera). — The  Construction  and 
Origin  of  the  Vertebral  Column.— Vertebral  Bodies  and  Vertebral 
Arches.— Skeleton-plate  and  Muscle-plate. — Formation  of  the  Skull 
from  the  Head-plates. — Gill-openings  and  Gill-arches. — Seuse-orgai-a. 
—Limbs. — The  Two  Front  Limbs  and  the  Two  Hind  Limbs. 

"  The  occurrence  of  an  internal  skeleton  in  definite  local  relations  to  the 
other  organ-systems,  and  the  articulation  of  the  body  into  homologous 
segments,  are  points  in  the  general  organization  of  Vertebrates  to  which 
especial  weight  must  be  given.  This  metameric  structure  is  more  or  less 
definitely  expressed  in  most  of  the  organs,  and  as  it  extends  to  the  axial 
skeleton,  the  latter  also  gradually  articulates  into  separate  segments,  the 
vertebrae.  The  latter,  however,  must  be  regarded  only  as  the 'partial  ex« 


MAIN   FEATURES   OF   MAMMALIAN   GERM-HISTORY.       329 

prossion  of  a  general  articulation  of  the  body,  which  is  all  the  more 
important  in  consequence  of  its  appearing  prior  to  the  articulation  of  the 
originally  inarticulate  axial  skeleton.  Hence  this  general  articulation  may 
be  considered  as  a  primitive  vertebral  structure,  to  which  the  articulation 
of  the  axial  skeleton  is  related  as  a  secondary  process  of  the  same  sort." — 
KA.KL  GEGENBAUK  (1870). 

THE  most  important  processes,  which  we  have  just  noticed 
in  the  construction  of  the  body  from  the  germ-layers,  are 
essentially  similar  in  all  Vertebrates.  In  these  points  Man 
entirely  resembles  the  other  Mammals ;  nor  do  the  latter 
essentially  differ  from  other  Vertebrates.  It  is  true  that  a 
more  exact  study  of  germ-history  brings  various  differences 
to  light,  some  of  which  are  very  striking:  among  these 
may  be  mentioned  the  formation  of  a  large  yelk-sac 
in  most  Fishes,  in  all  Reptiles,  Birds,  and  Mammals ;  also 
the  formation  of  the  amnion  and  allantois  in  the  three 
higher  vertebrate. classes.  But  all  these  remarkable  struc- 
tural conditions,  which  react  on  the  diversified  development 
of  other  parts,  were  only  kenogenetically  acquired  at  a  later 
stage,  in  consequence  of  Adaptation  to  the  conditions  of 
egg-life  ;  on  the  contrary,  the  most  important  conditions  of 
the  original  body-structure,  which  must  be  regarded  as 
palingenetic,  as  transmitted  by  Heredity  from  the  common 
parent-form  of  all  Vertebrates,  are,  on  the  whole  and  in  the 
main,  everywhere  the  same. 

As  such  essential  main  acts  in  the  germ-history  of  all 
Vertebrates,  the  following  must  be  especially  noted : — 1. 
The  formation  of  a  Gastrula  (in  the  most  original  form  in 
the  Amphioxus,  in  a  form  which  is  modified  from  the 
latter  in  all  other  Vertebrates).  2.  The  fission  of  the  four 
primary  germ-layers  into  four  secondary  germ-layers  (often 
with  a  three-layered  stage  intermediate  between  the  two 


330 


THE   EVOLUTION   OF   MAN. 


and  the  four-layered  stages).  3.  The  axial  soldering,  or 
the  coalescence  of  the  germ-layers  along  the  longitudinal 
axis  (giving  rise  to  the  axis-band).  4.  The  early  sepa- 
ration of  the  medullary  tube  from  the  skin-sensory  layer 
(by  the  formation  of  the  dorsal  furrow  and  the  spinal 
swellings).  5.  The  early  origin  of  the  primitive  kidney 
ducts  (probably  from  the  skin-sensory  layer).  6.  The  early 
division  of  the  skin-fibrous  layer  into  the  chorda,  the  primi- 
tive vertebral  cords,  and  the  trunk-muscle  plates.  7.  The 
separation  of  the  skin-fibrous  layer  from  the  intestinal- 
fibrous  layer  (giving  rise  to  the  body-cavity,  or  coeloma). 
8.  The  rudimentary  primitive  vessels,  or  aortae  (from  the 
intestinal-fibrous  layer).  These  important  germ-processes 
result  in  the  formation  of  ten  different  parts  of  the  body, 
which  we  may  call  "  the  primitive  organs,"  and  which,  in 
the  following  list,  are  represented  in  their  relation  to  the 
germ-layers.  (Of.  Fig.  99,  and  Plate  IV.  Fig.  3.) 


Phylogenetic  fission  of  the  germ-layers. 

Primitive  Organs 
(fig.  99). 

Ontogenetie  fission 
of  the 
germ-layers. 

I.  Secondary  germ- 
layer  : 

1.  Horn-plate  (A). 
2.  Medullary  plate 

A.  Upper  or 

Outer  primary  germ- 
layer  : 

Skin-sensory 
layer. 

3.  1'rimit'ive  kidney 

Sensory  layer, 
Remak. 

Skin-layer 

(Dt-rmal  layer,  or 
Exodtrma). 

II.  Secondary  germ-    [     4.  Chorda  (cV). 
layer:               I     5.  Primitive     vertP- 

Skin-flbrous      i    *tSS£&y& 

B.  Middle  or 

layer.           (         (hpl). 

Motor-germina1 

.HI.  Secondary  germ- 
B.                  |               layer  : 

7.  Body-cavity  (*p). 
8.  Intestinal   muscle 

tive  layer, 
Remak. 

laner  primary  germ-  \         Intestinal- 

plate  (df). 
9.  Primitive    aorta 

layer:            1    fibrous  layer. 

(ao\ 

Intestinal  layer  \  IV,  8eeoaAliry  germ-  /                                 \        c  Lo^erm 

glandular  layer. 


Trophic  layer, 
Remak. 


PRIMITIVE   ORGANS    OF   THE   VERTEBRATE.  331 

In  the  important  transverse  section  through  the  germ- 
shield  of  a  Chick  (Fig.  99),  which  represents  these  primitive 
organs  in  their  original  relative  positions,  they  are  seen  to 
be  flattened  and  spread  out ;  and  they  are  found  in  this 
same  condition  in  a  corresponding  transverse  section  through 
the  germ-shield  of  a  Mammal.  In  order  rightly  to  appre- 
ciate these  instructive  sections  (with  which  Figs.  3  and  4 
on  Plate  IV.  should  be  compared),  it  must  be  remembered 
that  the  layer-like  extension  of  the  flat  germ-layers  over 
the  surface  of  the  large  yelk-sac  represents  a  derived, 
kenogenetic  condition,  which  has  arisen  in  consequence  of 
the  gradual  acquisition  of  a  large  nutritive  yelk.  In  those 
low  Vertebrates  in  which  there  is  no  such  yelk-sac,  and  in 
which  the  original,  palingenetic  condition  is  more  or  less 


j.ft 


Fro.  99. — Transverse  section  through  the  germ-shield  of  a  Chick  (on  the 
second  day  of  incubation,  about  100  times  enlarged).  In  the  outer  germ- 
layer  the  axial  dorsal  furrow  has  completely  closed  and  forms  the  medullary 
tube  (mr),  which  has  separated  itself  from  the  horn-plate  (h).  In  the 
middle  germ-layer  the  axial  notochord  (c/i)  has  entirely  separated  itself 
from  the  two  primitive  vertebral  cords  (uw),  in  the  interior  of  which  a 
transitory  cavity  (uwh)  afterwards  forms.  The  side-layers  have  split  into 
the  outer  skin-fibrous  layer  (hpl)  and  the  inner  intestinal-fibrous  layer 
(df),  which  are  still  connected  by  the  middle  plates  (mp).  The  fissure 
(sp)  between  the  two  is  the  rudiment  of  the  body-cavity.  In  the  gap 
between  the  primitive  vertebral  cords  and  the  side-layers  on  either  side  are, 
attached  on  the  outer  side,  the  primitive  kidney  (ung),  on  the  inside  the 
primitive  artery  (ad).  (After  Kolliker. ) 
24 


332  THE  EVOLUTION   OF   MAN. 

retained,  the  germ-layers,  even  in  the  earliest  stage,  form 
closed  tubes,  which  may  be  immediately  referred  to  the 
tubular  shape  of  an  elongated  Gastrula.  (Of.  Figs.  62-69.) 

When,  therefore,  it  was  generally  thought  that  the 
main  object  of  the  germ-history  of  Vertebrates  was  to 
derive  the  later  organization  of  these  from  a  primitive, 
flat,  discoid  form,  the  two-layered  germ-disc  (or  the  three- 
layered  germ-shield),  a  grave  error  was  committed.91  For 
this  flat,  circular  germ-disc,  and  the  flat,  sole-shaped  germ- 
shield  which  arose  from  the  former,  are  phylogenetic  form- 
ations, which  arose  only  secondarily,  in  consequence  of  the 
accumulation  of  a  large  mass  of  nutritive  yelk  in  the 
primitive  intestine  of  the  primary  Gastrula ;  and  so  when, 
at  a  later  period,  the  dorsal  side  of  the  flat  germ-shield 
arches,  and  its  edges  bend  towards  each  other  and  coalesce 
into  tubes  on  the  ventral  side,  the  process  is  neither  primary 
nor  secondary,  but  tertiary. 

A  right  conception  of  the  formation  of  the  intestine  is 
evidently  the  real  point  on  which  a  thorough  knowledge  of 
these  important  germinal  processes  depends.  The  greatest 
difficulties  are  solved  when  a  clear  and  correct  conception 
of  the  formation  of  the  intestinal  canal  has  been  acquired. 
For  the  primitive  intestine  is,  according  to  the  Gastnea 
Theory,  the  earliest  and  the  most  important  organ  of  the 
animal  body.  In  order  to  gain  this  clear  idea  of  the  forma- 
tion of  the  intestinal  tube  and  the  parts  attached  to  it,  it  is 
especially  necessary  to  note  accurately  the  important  modi- 
fication undergone  by  the  intestinal-glandular  layer  of  the 
mammalian  germ.  This,  as  has  been  said,  is  at  first  a 
simple  layer  of  cells  (an  epithelium),  which  lines  the  inner 
surface  of  the  globular  intestinal  germ-vesicle.  It  is  a 


MODIFICATION   OF   INTESTINAL-GLANDULAR   LAYER.      333 

simple  globule,  the  wall  of  which  consists  of  a  simple  layer 
of  homogeneous  cells  (Fig.  100,  A  dd}.      The  first  change  in 


FIG.  100.— -The  separation  of  the  discoidal  mammalian  germ  from  the 
yelk  sac,  seen  in  section  (diagrammatic).  A.  The  gei-m-disc  (h,  hf)  lies  ex- 
tended  on  one  side  of  the  intestinalgerm- vesicle  (kb).  B.  In  the  centre  of  the 
germ-disc  the  medullary  furrow  (mr),  and  under  that  the  notochord  (ch) 
appear.  (7.  The  intestinal. fibrous  layer  (<!/)  has  grown  round  the  intestinal- 
glandular  layer  (dd).  D.  Skin-fibrous  layer  (hf)  and  intestinal-fibrous  layer 
(df)  part  round  the  circumference  of  the  germ-disc  ;  the  intestine  (d)  begins 
to  separate  itself  from  the  yelk-sac  or  navel-vesicle  (nb).  E.  The  intestinal 
tube  (mr)  is  closed;  the  body-cavity  (c)  begins  to  form.  F.  The  primithe 
vertebrae  (?t1)  appear ;  the  intestine  (d)  is  almost  completely  closed.  G. 
The  primitive  vertebrae  (w)  begin  to  grow  round  the  medullary  tube  (mr) 
and  the  notochord  (ch)  ;  the  intestine  (d)  is  separated  from  the  navel, 
vesicle  (nb).  H.  The  vertebras  (w)  have  enclosed  the  medullary  tube  (mr) 
and  the  notochord  (ch)  ;  the  body-cavity  (c)  is  closed  ;  the  navel-vesicle  has 
disappeared.  The  amnion  and  serous  membranes  are  omitted. 

In  all,  the  letters  indicate  the  same  parts  :  h,  horn-plate ;  mr,  medullary- 
tube  ;  hf,  skin-fibrous  layer  ;  w,  primitive  vertebrae  ;  ch,  notochord  ;  c,  body- 
cavity  ;  d/,  intestinal-fibrous  layer ;  dd,  intestinal-glandular  layer ;  d,  in- 
testinal cavity  ;  nb,  navel-vesicle. 


334  THE   EVOLUTION    OF   MAX. 

this  globular  formation  is  that  at  one  point  in  the  germ 
disc,  immediately  below  the  notochord,  and,  therefore,  below 
the  axis  of  the  developing  body,  a  furrow-like  depression 
arises.  This  is  the  primitive  groove  (Fig.  100,5).  It  gradually 
becomes  deeper  and  broader,  assumes  the  form  of  a  canal, 
and  completely  separates  from  the  germ-vesicle,  of  which 
it  originally  formed  a  part  (Fig.  100,  D — H}.  At  first 
the  whole  intestinal  germ-vesicle  is,  in  a  certain  sense,  the 
intestinal  cavity.  We  may,  therefore,  compare  the  entire 
intestinal  germ  vesicle  of  the  Mammal,  the  wall  of  which, 
closed  on  all  sides,  is  formed  by  the  intestinal  layer,  with  the 
primitive  intestine  of  a  Gastrula,  the  primitive  mouth  of 
which  has  closed.  This  primitive  intestine  separates  into 
two  parts,  the  permanent  after-intestine  (d),  and  the  tran- 
sient navel-vesicle  (nb}. 

This  is  also  true  of  the  formation  of  the  intestine  in 
Birds  and  Reptiles.  For  in  these,  the  large  yelk-sac,  filled 
with  nutritive  yelk,  represents  the  smaller  mammalian 
navel-vesicle,  filled  with  clear  liquid.  In  Birds  and  Reptiles 
again,  the  later,  permanent  intestine  also  separates  itself 
from  the  yelk-sac  by  the  intestinal  groove  changing  into  a 
canal,  into  the  intestinal  tube.  This  tube  is  formed  from 
the  intestinal-furrow  in  the  same  way  as  the  medullary 
tube  originates  from  the  dorsal  furrow.  The  groove  grows 
deeper  and  deeper ;  its  edges  grow  downwards  towards  each 
other,  and  coalesce  at  the  point  at  which  they  meet.  But 
the  difference  between  the  structure  of  the  intestinal  tube 
and  that  of  the  medullary  tube  consists,  as  we  have  shown 
in  the  fact  that  the  medullary  tube  is  closed  equally  along 
its  whole  length  in  a  suture,  while  the  intestinal  tube 
together  more  concentrically,  not  only  from  the  two 


THE   TWO    INTESTINAL   CAVITIES.  335 

edges,  but  the  ends  also  come  together  with  the  edges  which 
close,  and  form  a  navel. 

With  this  concentric  closing  of  the  intestinal  tube  is 
connected  the  formation  of  two  cavities,  which  are  called 
the  head  intestinal  cavity  and  the  pelvic  intestinal  cavity. 
When  the  embryo  gradually  becomes  detached  from  the 
wall  of  the  germ-vesicle,  on  which  it  at  first  lies  flat,  the 
anteiior  and  posterior  ends  are  the  first  to  be  released, 
while  the  central  portion  of  the  ventral  surface  continues 
attached  to  the  yelk-sac  by  the  yelk-duct,  or  navel-duct 
(Fig.  101,  m).  In  the  mean  time  the  dorsal  surface  of  the 
body  becomes  much  arched;  the  head  end,  on  the  other 
hand,  bends  downward  and  against  the  breast,  while  the 
tail  end,  in  the  same  way,  presses  against  the  abdomen ; 
the  embryo  tries  to  roll  itself  together,  as  a  hedgehog 
makes  itself  into  a  ball  to  ward  off  its  enemies.  This  arch- 
ing of  the  back  is  caused  by  the  quicker  growth  of  the 
dorsal  surface,  and  is  directly  connected  with  the  detach- 
meut  of  the  embryo  from  the  yelk-sac  (Fig.  101).  In  the 
head  there  is  no  separation  between  the  skin-fibrous  layer 
and  the  intestinal-fibrous  layer,  as  is  the  case  in  the  trunk, 
but  the  two  layers  remain  attached  and  form  the  so-called 
"head-plates."  Now  as  these  head-plates  free  themselves 
at  a  very  early  period  from  the  surface  of  the  germ-area, 
and  grow,  first  downward  toward  the  surface  of  the 
intestinal  germ-vesicle,  and  then  backwards  toward  the 
point,  at  which  the  latter  passes  into  the  intestinal  groove ; 
a  rfmall  cavity  is  thus  formed  within  the  head  portion,  which 
represents  the  foremost  blind  end  of  the  intestine.  This 
is  the  small  head  intestinal  cavity  (Fig.  102,  to  the  left 
of  d};  its  opening  into  the  middle  intestine  is  called  the 


336 


THE    EVOLUTION   OF   MAN. 


anterior  "  intestinal  gate "  (Fig.  102,  at  d).  Just  in  the 
same  way  the  tail  curves  back  against  the  ventral  surface ; 
the  intestinal  wall  then  encloses  posteriorly  a  similar  small 


r  ff 


FIG.  101.— Longitudinal  section  through  the  embryo  of  a  Chick  (fifteenth 
day  of  incubation).  Embryo  with  arched  dorsal  surface  (black)  :  d,  intes- 
tine ;  o,  mouth ;  a,  anus ;  I,  lungs ;  h,  liver ;  g,  mesentery ;  v,  auricle  of 
heart ;  fe,  ventricle  of  heart ;  b,  arterial  arches ;  t,  aorta ;  c,  yelk-sac ;  m, 
yelk-duct ;  u,  allantois ;  r,  stalk  of  allantois ;  n,  amnion  ;  w,  amnion- 
cavity  ;  s,  serous  membrane.  (After  Baer.) 

cavity,  the  hind  end  of  which  is  blind ;  this  is  the  pelvic 
intestinal  cavity.  Its  opening  into  the  middle  intestine 
is  the  "  hind  intestinal  gate." 

In  consequence  of  these  processes  the  embryo  assumes  a 
form  resembling  a  canoe  lying  bottom  upward.  Imagine  a 
canoe  with  rounded  ends,  and  fitted  with  a  little  deck  fore 
and  aft;  then  turn  this  canoe  upside  down,  so  that  its 
arched  bottom  is  uppermost:  this  affords  an  approximate 
representation  of  this  canoe-shaped  embryo  (Fig.  101,  e\ 


THE    GERM-VESICLE   BECOMES   THE   YELK-SAC.  337 

The  reversed  convex  keel  represents  the  middle  line  of  the 
back ;  the  little  chamber  under  the  fore-deck  represents 
the  head  intestinal  cavity,  and  that  under  the  after-deck 
the  pelvic  intestinal  cavity.  (Of.  Fig.  94,  p.  312.) 


FIG.  102. — Longitudinal  section  through  the  front  half  of  a  chick 
(at  the  end  of  the  first  day  of  incubation),  seen  from  the  left  side  :  k,  head- 
plates ;  ch,  notochord ;  above  the  latter,  the  blind  front  end  of  the 
medullary  tube  (mr)  ;  below  it  the  head  intestinal  cavity,  the  blind  front 
end  of  the  intestinal  tube;  d,  intestinal-glandular  layer;  df,  intestinal- 
fibrous  layer;  h,  horn-plate;  hh,  heart-cavity;  hk,  hear t -cap ;  ks,  head- 
sheath  ;  kk,  head-cap.  (After  Remak.) 

With  its  two  free  ends  the  embryo  now  presses 
somewhat  into  the  external  surface  of  the  germ-vesicle, 
and  at  the  same  time  lifts  the  middle  portion  away  from  the 
germ-vesicle.  The  consequence  is  that  the  germ-vesicle 
soon  appears  to  be  merely  a  pouch-shaped  appendage  pro- 
truding from  the  middle  of  the  body.  This  appendage,  which 
continues  to  decrease  in  size,  is  afterwards  called  the  yelk- 
sac,  or  navel-vesicle.  (Of.  Fig.  94,  4,  6,  ds;  Fig.  100,  and  Plate 
V.  Fig.  14.)  The  cavity  of  this  yelk-sac,  or  cavity  of  the 
germ-vesicle,  communicates  with  the  growing  intestinal 
cavity  through  a  wide  connecting  aperture,  which  after- 


338  THE   EVOLUTION   OF   MAN. 

wards  extends  into  a  long  narrow  canal,  the  yelk-duct. 
Let  us  suppose  we  are  within  the  cavity  of  the  yelk-sac ; 
we  may  then  pass  from  it,  through  the  yelk-duct  (Fig.  101,  m], 
directly  into  the  middle  part  of  the  intestinal  canal,  which 
is  still  wide  open.  If  from  there  we  pass  on  into  the 
head  portion  of  the  embyro,  we  reach  the  head  intes- 
tinal cavity,  the  anterior  end  of  which  is  blind.  If,  on  the 
other  hand,  we  pass  from  the  middle  of  the  intestine  back- 
wards into  the  tail  portion,  we  reach  the  pelvic  intestinal 
cavity,  the  hind  end  of  which  is  blind  (Fig.  94,  3).  The 
first  rudiment  of  the  intestinal  tube  now  consists,  therefore, 
strictly  speaking,  of  three  distinct  sections  :  (1)  the  head 
intestinal  cavity,  the  hind  end  of  which  opens,  through  the 
front  intestinal  gate,  into  the  middle  intestine ;  (2)  the 
middle  intestinal  cavity  which  opens  downwards,  through 
the  yelk-duct,  into  the  yelk-sac ;  and  (3)  the  pelvic  intes- 
tinal cavity,  the  front  of  which  opens,  through  the  posterior 
intestinal  gate,  into  the  middle  intestine. 

At  first  the  mouth  and  anal  openings  are  wanting. 
The  whole  primitive  intestinal  cavity  is  entirely  closed,  and 
is  only  connected  in  the  middle  by  the  yelk-duct  with  the 
cavity  of  the  intestinal  germ-vesicle,  which  is  also  closed 
(Fig.  94,  3).  The  two  future  openings  of  the  intestinal 
canal,  the  anal  opening  and  the  mouth-opening,  form  only 
secondarily,  on  the  outside,  and  from  the  outer  skin; 
that  is  to  say,  a  groove-like  depression  arises  in  the  horn- 
plate  at  the  point  where  the  mouth  is  afterwards  situated, 
and  this  grows  deeper  and  deeper,  growing  towards  the 
blind  front-end  of  the  head  intestinal  cavity :  this  is  the 
mouth-groove.  A  similar  groove-like  depression  appears 
posteriorly  on  the  outer  skin,  at  the  point  where  the  anus 


ORIGIN   OF   THE   MOUTH   AND   ANUS.  339 

is  afterwards  found,  and  this  also  grows  continually  deeper 
and  towards  the  blind  anterior  end  of  the  pelvic  intestinal 
cavity ;  this  is  the  anal  groove.  At  length  the  innermost 
and  deepest  parts  of  these  grooves  touch  the  two  blind  ends 
of  the  primitive  intestinal  canal,  from  which  they  are  now 
only  separated  by  a  thin  membranous  partition  wall 
Finally,  this  thin  skin  is  broken  through,  and  the  intestinal 
tube  now  opens  outward  in  front  through  the  mouth- 
opening,  and  in  the  rear  through  the  anal  opening  (Fig. 
94,  4  ;  101).  At  first,  then,  we  really  have  before  us,  if  we 
look  into  these  grooves,  a  partition  wall  separating  them 
from  the  cavity  of  the  intestinal  canal,  and  it  is  only  later 
that  these  partitions  disappear.  The  mouth  and  anal 
openings  develop  secondarily. 

The  remnant  of  the  intestinal  germ-vesicle,  which  we 
have  called  the  navel-vesicle,  or  yelk-sac,  grows  smaller  and 
smaller  as  the  intestine  develops,  and  finally  hangs  like  a 
small  pouch  from  the  middle  of  the  intestine  by  a  slender 
stalk,  by  the  yelk-duct  (Fig.  94-,  5  ds).  This  yelk-duct  is 
of  no  permanent  importance,  and,  like  the  yelk-sac  itself,  is 
completely  degraded  and  absorbed.  Its  contents  are  absorbed 
by  the  intestine,  and  the  yelk-duct  itself  closes,  ,The  place 
at  which  it  attaches  itself  to  the  navel  is  the  intestinal 
navel.  The  complete  closing  of  the  intestine  finally  takes 
place  at  this  spot.  (Of.  Chap.  XII.,  and  Plate  V.  Fig. 
U,  15.) 

Just  as  the  intestinal  tube  arose  from  the  vegetative 
germ-layer,  so  from  the  animal  germ-layer  arises  the  outer 
ventral  wall,  which  surrounds  the  entire  body-cavity 
(cwloma),  and  includes  the  intestine.  It  develops  from  the 
outer  portions  of  the  side-layers.  As  has  been  already 


34O  THE   EVOLUTION   OF    MAN. 

observed,  these  side-layers,  which  for  a  time  were  separated 
from  the  primitive  vertebral  cords,  afterwards  again  adhere 
to  the  latter.  While  the  inner  portion  of  the  side-layers 
(belonging  to  the  intestinal-fibrous  layer)  is  thus  forming 
the  external  wall  of  the  intestine,  the  outer  portion  of  the 
same  layers  (belonging  to  the  skin-fibrous  layer)  grows  in 
a  circle  round  the  intestine,  thus  closing  the  body-cavity 
(Fig.  ]  00,  p.  333).  The  edges  of  the  ventral  plates,  as  these 
portions  of  the  side-layers  are  called,  grow  toward  each 
other  from  all  sides,  continually  narrowing  the  slit-like 
ventral  opening,  from  which  the  yelk-sac  depends.  Finally, 
the  latter  is,  in  Mammals,  completely  separated  from 
the  intestine  by  the  closing  of  the  ventral  plates,  while  in 
Birds  and  Reptiles  it  is  taken  into  the  intestine.  This  point 
at  which  the  ventral  wall  finally  closes — the  last  point  of 
coalescence — is  the  ventral  navel,  the  externally  visible  skin- 
navel,  which  is  commonly  briefly  called  the  navel.  This 
must  be  distinguished  from  the  inner,  or  intestinal  navel, 
which  is  the  point  at  which  the  intestinal  canal  closes,  and 
of  which  no  trace  can  afterwards  be  found.  With  the 
closing  of  the  intestinal  tube  and  of  the  ventral  wall, 
the  double  tubular  form  of  the  vertebrate  body  is  com- 
plete. 

A  few  words  must  still  be  said  concerning  the  modifica- 
tions which,  while  these  processes  are  going  on,  take  place 
in  the  primitive  kidneys  and  in  the  blood-vessels.  The 
primitive  kidneys,  which  at  first  lie  quite  superficially  just 
below  the  outer  skin  (epidermis,  Fig.  99,  ung),  soon  penetrate 
far  into  the  interior  in  consequence  of  peculiar  conditions  of 
growth  (Figs.  95,  96,  ung,  pp.  317, 318) ;  at  last  they  lie  very 
far  within,  underneath  the  chorda  dorsalis  (Fig.  97, un,  p.  318) 


MODIFICATIONS   OF  THE   VASCULAR   SYSTEM.  341 

Similarly  the  two  primitive  aortae  penetrate  within, 
below  the  notochord,  and  there  eventually  amalgamate 
and  form  a  single  secondary  aorta,  which  is  situated  under 
the  rudimentary  vertebral  column.  (Cf.  Figs.  95-98,  ao.) 
So,  too,  the  cardinal  veins,  the  first  rudiments  of  the 
venous  blood-vessels,  penetrate  further  inwards,  and  after- 
wards lie  directly  over  the  primitive  kidneys  (Fig.  97,  vc\ 
In  the  same  locality,  at  the  inner  side  of  the  primitive 
kidneys,  the  first  rudiments  of  the  sexual  organs  soon 
become  visible.  The  chief  portion  of  this  apparatus,  apart 
from  all  its  appendages,  is,  in  the  female,  the  ovary — in  the 
male,  the  testes.  Originally  both  these  appear  in  the  form 
of  a  simple  hermaphrodite  gland,  formed  from  a  small  por- 
tion of  the  coelom-epithelium,  the  cellular  lining  of  the  body- 
cavity,  at  the  point  of  contact  between  the  skin-fibrous  layer 
and  the  intestinal-fibrous  layer.  It  is  only  secondarily  that 
this  hermaphrodite  gland  seems  to  connect  itself  with  the 
primitive  kidney  ducts,  which  lie  very  close  to  them,  and 
which  are  very  importantly  related  to  the  sexual  glands. 
(Cf.  Chap.  XXV.,  and  Plate  IV.  Figs.  5-7.) 

We  will  now  leave  the  transverse  sections  of  the  verte- 
brate body,  the  comparison  of  which  has  been  so  ex- 
tremely instructive  and  important,  and  by  means  of  which 
we  have  solved  the  hardest  problem  of  germ-history,  the 
problem  as  to  the  part  taken  by  the  germ-layers  in  the 
formation  of  the  body.  In  place  of  those,  we  will  now 
examine  the  longitudinal  form  of  the  rudimentary  embryo 
of  the  mammalian  body,  partly  superficially,  and  partly  in 
various  longitudinal  sections. 

Let  us  now  examine  the  surface,  from  the  dorsal  side,  of 
that  very  simple  embryonic  form  which  we  called  the  sole- 


THE    EVOLUTION   OF   MAX.- 


f,       f  .        .' 

, 
___  /^rriSflf.- 


FIGS.  103-5. — Lyre-shaped  germ-shield  of  a  Chick,  in  three  consecutive 
stages  of  development,  seen  from  the  dorsal  surface  (about  20  times  the 
natural  size).  Fig.  103,  with  six  pairs  of  primitive  vertebrae.  Brain  is  a 
simple  bladder  (hb).  The  spinal  furrow  is  open  behind  a>;  at  the  posterior 
end  it  is  very  wide  open  at  z ;  mp,  medullary  plates ;  sp,  side-plates ;  y, 
limit  between  throat-cavity  (sh)  and  head -intestine  (vd). — Fig.  104,  with 
10  pairs  of  primitive  vertebrae.  The  brain  has  parted  into  three  bladders  : 
v,  fore-brain  ;  m,  mid-brain  ;  h,  hind-brain  ;  c,  heart ;  dv,  yelk-veins.  Tho 
medullary  furrow  continues  wide  open  at  the  posterior  end  («)  ;  mp,  medul- 
lary plates. — Fig.  105,  with  16  pairs  of  primitive  vertebrae.  The  brain  has 
parted  into  five  bladders  :  v,  fore-brain ;  z,  twixt-brain  ;  m,  mid-brain  ;  h, 
hind-brain ;  n,  after-brain ;  a,  eye-bladders ;  g,  ear-vesicles ;  c,  heart ;  dv, 
yelk-veins ;  mp,  medullary  plates ;  uw,  primitive  vertebras. 


RUDIMENT   OF  THE   BRAIN.  343 

shaped  or  lyre-shaped  germ-shield  (Figs.  86,  87,  p.  298).  In 
the  middle  line  of  its  dorsal  surface  the  primitive  groove 
first  made  its  appearance,  enclosed  by  the  two  parallel 
dorsal,  or  medullary  swellings.  The  coalescence  of  these 
formed  the  medullary  tube.  When  we  examine  the  further 
modifications  of  this,  we  very  soon  perceive  a  difference 
between  the  formation  of  the  anterior  and  that  of  the 
posterior  ends.  At  the  anterior  end  in  Man,  as  in  all  the 
higher  Vertebrates,  the  brain  very  soon  begins  to  separate 
or  differentiate  from  the  medullary  tube.  The  first  rudi- 
ment of  the  brain  is  merely  a  roundish,  bladder-like  pro- 
tuberance of  the  vertebral  canal  (Fig.  103,  kb).  Very  soon, 
however,  this  bladder  is  divided  by  two  circular  contrac- 
tions of  its  circumference,  into  three  consecutive  vesicles, 
the  so-called  primitive  brain  bladders  (Fig.  104,  v  m  k). 
Two  other  similar  contractions  then  appear,  so  that  we  now 
find  five  brain-bladders  in  a  row  (Fig.  105).  This  is  the 
mode  of  development  of  the  brain  in  all  Mammals,  from  the 
simplest  Fishes  to  Man.  In  all,  we  find  a  simple  vesicle  as 
the  first  rudiment  of  the  brain,  which  is  afterwards  parted, 
by  contractions  in  its  circumference,  into  five  smaller 
bladders.  Though  the  brain,  as  the  organ  of  the  soul  and 
the  mental  activities,  afterwards  develops  in  various  Verte- 
brates in  such  very  various  ways,  yet  the  first  rudiment 
is  of  this  simple  and  homogeneous  form.  This  is  a  fact  of 
the  highest  importance. 

Directly  below  the  medullary  tube,  in  the  lyre-shaped 
germ-shield,  we  found  the  notochord.  Right  and  left  of  the 
notochord  the  two  parallel  primitive  vertebral  cords  had 
split  away  from  the  side-layers.  But  while  the  five  brain- 
bladders  are  becoming  distinct  at  the  anterior  end  of  the 


344 


THE   EVOLUTION   OF   MAN. 


medullary  tube,  the  two  primitive  vertebral  cords  in  the 
centre  of  the  primitive  germ  break  up  into  a  number  of 
pieces,  lying  one  behind  another,  and  resembling  small 
cubes  on  each  side  of  the  medullary  tube.  Two  pairs 
usually  first  make  their  appearance  simultaneously.  Then 


Fro.  106-109. —The  Germ- 
disc  of  a  Rabbit  (the  circular 
germ -area  with  the  lyre- 
shaped  germ-shield),  seen 
from  the  dorsal  surface,  in 
four  consecutive  stages  of 
evolution  (about  ten  times 
the  natural  size).  (After 
Bischoff.) 

In  Fig.  106  the  embryo  (b) 
is  as  yet  without  primitive 
vertebrae ;  the  open  dorsal 
furrow  (a)  surrounded  by 
a  narrow  light  germ-area 
(a.  pellucida,  a),  in  the 
middle  of  the  dark  gertn- 
area  (a.  opaca,  d). 


In  Fig.  107  seven  pri. 
niitive  vertebrae  (c)  may 
already  be  seen;  the  dorsal 
furrow  is  closed;  the  first 
rudiment  of  the  brain  (a), 
a  brain -bladder,  behind 
which  a  second  (6)  is  form- 
ing, is  arising ;  the  light 
germ -area  is  now  only 
visible  at  the  anterior  end, 
in  the;  form  of  a  dark  sickle- 
shaped  body  on  a  black 
ground. 


ORIGIN   OF   THE   PRIMITIVE   VERTEBRAE. 


345 


appear  three,  four,  and  five  pairs,  and  finally  a  larger  number 
of  these  pieces,  which  are  called  the  primitive  vertebrae. 


In  Fig.  108  the  em- 
bryo  has  eight  primi- 
tive vertebrae  and 
three  brain-bladders  ; 
the  first  brain-bladder 
(b)  shows  two  lateral 
protuberances,  the 
first  rudiments  of  the 
eye-bladders  (c)  ;  the 
second  (cZ)  and  the 
third  (e)  brain-blad- 
ders are  much 
smaller;  a  indicates 
the  edge  of  the  head- 
shciitli  of  the  amnion. 


In  Fig.  109  the 
embryo  has  ten 
primitive  verte- 
bras; in  the  germ- 
area  the  first 
traces  of  the  net 
of  blood-vessels 
appear,  bounded 
by  the  vena  termi- 
nalis  (a)  :  b,  tail- 
sheath,  bb,  head- 
sheath  of  the 
amnion;  the  folds 
in  the  latter  indi- 
cate the  serous 
membrane. 


346  THE    EVOLUTION    OF   MAN. 

In  Fig.  107  there  are  seven,  in  Fig.  108  there  arc 
sight,  and  in  Fig.  109  ten  pairs  of  primitive  vertebrae.  Their 
number  afterwards  increases  considerably,  amounting  in 
Man  to  upwards  of  thirty.  As  we  shall  presently  see,  out 
of  each  pair  of  these  primitive  vertebral  segments  an  indi 
vidual  section  of  the  trunk,  a  metameron,  develops.  Each 
pair  of  primitive  vertebrae  is  not,  as  the  name  seems  to 
indicate,  merely  the  rudiment  of  a  future  vertebra,  but,  in 
addition  to  the  vertebra,  the  appropriate  muscles  also 
develop  from  it,  as  does  a  pair  of  nerve-roots,  etc.  It  is 
only  the  innermost  portion  of  the  primitive  vertebra,  the 
part  lying  next  to  the  notochord,  that  gives  rise  to  the 
rudiment  of  the  articulated  vertebral  column,  extending 
from  the  cranium  to  the  tail,  and  composed  of  a  number  of 
bony  vertebral  rings.98 

The  breaking  up  of  the  vertebral  cord  into  a  double 
chain  of  primitive  vertebral  segments,  or,  briefly,  the 
forming  of  the  metamera,  is  of  the  greatest  importance, 
because  it  is  in  this  process  that  the  body  of  the  Vertebrate 
passes  from  its  originally  inarticulate  to  its  permanent 
articulate  conditions.  The  developed  Vertebrate  is  composed 
of  a  chain  of  homogeneous  parts,  lying  one  behind  another 
precisely  as  are  the  Articulated  Animals  (Arthropoda). 
In  the  latter  class,  in  Crabs,  Spiders,  Millipedes,  and  insects, 
this  articulation  is  very  clearly  marked  externally ,  the 
skin  between  each  two  members  (metamera)  having  a  ring- 
shaped  contraction  or  dent  round  the  circumference  of  the 
body.  In  Vertebrates  the  articulation  of  the  body  is  equally 
complete,  but  it  does  not  appear  externally,  though  internally 
it  is  fundamental  Every  Vertebrate,  in  its  perfect  state,  is 
ail  articulated  person.  Its  personality  forms  a  chain  of 


ARTICULATION.  347 

members,  metamera,  or  trunk-segments.  In  the  same  way 
in  which  the  articulate  and  the  externally  articulated 
Worms  developed  from  an  inarticulate  condition,  so  the 
internally  articulated  Vertebrate  proceeded  from  an 
originally  inarticulate  condition.  We  shall  presently  ex- 
amine more  closely  the  living  representative  of  this  con- 
dition, the  Ascidia,  a  remarkable  class  of  inarticulate  Worm 
forms.  (Chapters  XIII.  and  XIV.) 

This  process  of  articulation  or  metameric  formation  is,  I 
repeat,  of  the  highest  importance  in  enabling  us  to  iinder- 
stand  each  higher  animal  form,  not  only  in  its  morpho- 
logical, but  also  in  its  physiological  relations.  This  articu- 
lation is  one  of  the  most  important  conditions  necessary 
to  perfection :  it  is  one  of  the  principal  causes  of  the 
complex  body-functions  of  higher  animals.  The  inarticulate 
animal  can  never  attain  so  high  a  degree  of  perfection  in 
form  or  in  function  as  the  articulated.  And  the  reason  is 
plain.  These  members,  or  metamera,  are,  in  a  certain  sense, 
independent  individuals.  By  division  of  labour,  these 
originally  homogeneous  individuals  develop  into  the  different 
parts  of  the  composite  body-person,  just  as  the  embryonic 
cells  fashion  themselves,  in  consequence  of  division  of  labour, 
into  the  various  tissues.  The  body  of  articulated  animals 
may  be  likened  to  a  railway  train,  in  which  the  individual 
carriages,  held  together  by  the  couplings,  represent  the 
metamera.  The  engine  is  the  head  of  this  articulated 
organism.  Then  come  tender,  mail-van,  luggage-vans, 
passenger-carriages,  cattle-trucks,  etc.  Each  separate 
waggon  or  carriage  is  morphologically  an  individual,  and 
physiologically,  yet  the  entire  chain  presents  only  a  single 
individual,  the  railway  train.  As  in  this  instance  tne 
25 


348  THE   EVOLUTION   OF  MAN. 

various  functions  are  distributed  among  the  various  kinds  of 
carriages  —  functions  which  each  separate  carriage  can- 
not discharge  simultaneously — so  in  the  articulated  animal 
body  the  division  of  labour  among  the  metamera  of  the 
trunk  must  be  regarded  as  a  material  advance. 

The  best  explanation  of  the  nature  of  metameric 
formation  is  afforded  by  the  articulated  Worms,  especially 
the  Tape-worms  and  the  Ringed-worms  (Annelida).  In 
these  the  members,  or  metamera,  composing  the  ringed  body, 
are  all  of  the  same  structure  and  of  the  same  form-value. 
The  first  member,  the  head,  alone  seems  to  be  differently 
formed  and  more  or  less  differentiated.  In  many  Tape- 
worms the  various  members  are  so  independent,  that  many 
zoologists  regard  each  separate  metameron  as  an  individual 
animal,  and  the  whole  chain  of  members  as  a  colony  o. 
animals.  In  a  certain  sense  this  is  quite  correct,  in  so  far 
as  each  separate  metameron  is  an  individual  of  a  lower 
order,  while  the  chain,  composed  of  many  metamera,  is  an 
individual  of  a  higher  order.  But  in  proportion  as  the 
separate  members  relinquish  their  independence ;  in  .pro- 
portion as  they  become  differentiated  in  consequence  oi 
division  of  labour,  and  become  dependent  on  each  other 
and  on  the  whole  body,  and  in  proportion  as  the  latter 
becomes  centralized,  the  more  perfect  does  the  entire 
unitary  organism  become.  In  most  Articulated  Animals 
(Arthropodct),  and  in  all  Vertebrates,  centralization  has  so 
far  progressed  that  the  individual  metamera  are  no  longer 
of  any  importance  in  themselves  alone,  and  are  to  be  con- 
sidered merely  as  the  necessary  component  parts  of  the 
entire  chain. 

When  we  investigate  the  origin  of  the  metameric  chain 


GROWTH    OF   THE   METAMERA.  349 

in  the  Worms,  we  find  that  it  results,  in  consequence  of 
repeated  asexual  generative  processes,  in  consequence  of 
what  is  called  terminal  budding,  from  an  originally  inarticu- 
late Worm-body,  which  is  equivalent  to  a  single  metameron. 
Thus  the  Tape- worm  embryo  is  at  first  all  head  and  on 
this  head,  which  is  only  equivalent  to  a  single  metameron, 
repeated  budding  gives  rise  to  one  metameron  after  another; 
all,  however,  remain  connected.  So,  too,  in  the  Ringed 
Worms  (Annelida)  the  originally  inarticulate  body  puts  out 
numerous  buds  from  its  posterior  extremity,  thus  giving 
rise  to  the  long  articulated  chain.  Such  is  the  nature  of 
this  process,  which,  however,  in  the  germ-history  of  Articu- 
lated Animals  and  Vertebrates  appears  much  compressed 
and  secondarily  modified.  Originally,  however,  every  ver- 
tebrate animal  is  just  such  a  metameric  chain,  which  has 
arisen,  in  consequence  of  terminal  budding,  from  an  in- 
articulated  germ." 

As  the  metamera  arise  in  this  way,  it  will  readily  be 
understood  that  the  anterior  primitive  vertebrae  are  earliest 
found.  Such  is  indeed  the  case.  The  earliest  primitive 
vertebrae,  which  are  situated  about  the  centre  of  the  germ, 
are  the  first  and  second  neck- vertebrae.  Then  come  the 
third  and  fourth  neck-vertebrae,  and  so  on.  Each  primitive 
vertebral  segment  in  its  turn  soon  produces,  by  the  process 
of  budding,  a  new  metameron  at  its  posterior  extremity, 
till  the  chain  is  complete.  The  entire  jointed  body  grows, 
therefore,  in  a  direction  from  front  to  rear.  In  this  way 
the  articulated  vertebral  column  of  Man  is  at  length  pro- 
duced (Figs.  110,  111).  In  the  developed  Man  it  is  composed 
of  the  cranium,  with  a  chain  of  thirty -three  or  thirty-four 
different  vertebrae  :  viz.,  seven  neck-vertebrse,  twelve  chest- 


350  THE   EVOLUTION   OF   MAN. 

vertebrae,  to  which  the  ribs  are  attached,  five  lumbar- 
vertebrae,  five  cross-vertebrae  (inserted  into  the  pelvis),  and 
four  or  five  tail-vertebras.  Each  of  these  represents  a 
corresponding  section  of  the  nervous,  muscular,  and  vascular 
systems,  eta 

A  further  consequence  of  the  mode  of  development  of 
the  metamera  is,  that  nearly  the  whole  front  half  of 
the  lyre-shaped  germ-shield  (Figs.  103,  107)  mustre  present 
the  future  head.  The  seven  primitive  vertebras  which 
occupy  the  third  quarter  of  the  whole  length,  form  the  neck, 
so  that  all  the  rest  of  the  body  originates  from  only  the 
fourth  and  last  quarter.  This  proportion  seems  strange  at 
first,  but  its  phylogenetic  explanation,  as  the  result  of  the 
terminal  budding,  is  simple.  The  head  portion  of  the 
vertebrate  animal  must  accordingly  be  regarded  phyloge- 
netically  and  originally  as  the  oldest  portion  of  the  body — 
as  a  group  of  a  few  (six  to  ten)  closely  coalescent  metamera, 
which,  by  continued  budding  at  the  posterior  extremity, 
have  produced  the  remainder  of  the  body:  The  tail,  on 
the  other  hand,  is  the  most  recent  part,  the  latest  in  order 
of  development. 

As  has  been  already  observed,  the  articulation  affects 
the  entire  body  of  the  Vertebrate,  although  the  skin  shows 
no  external  signs  of  articulation.  The  primitive  vertebral 
pieces  are,  therefore,  not  merely  rudiments  of  future 
vertebras ;  they  are  real  metamera,  or  trunk-segments, 
Each  metameron  first  appears  as  a  nearly  cube-shaped, 
sol  iti,  roundly-hexagonal  body,  entirely  composed  of  cells. 

FIG.  110. — Human  skeleton,  from  the  front. 

Fig.  111. — Human  skeleton,  from  the  right  side.  The  arms  have  been 
removed.  (Both  figures  after  H.  Meyer.) 


352 


THE   EVOLUTION   OF  MAN. 


These  cells  are  all  the  products  of  the  skin-fibrous  layer. 
At  a  very  early  period,  a  small  cavity  appears  in  each  of 
these  solid  primitive  vertebrae,  which  cavity,  however,  soon 
again  disappears.  This  "  primitive  vertebral  cavity  "  (Figs. 
95,  96,  uwh,  pp.  317, 318)  is  worthy  of  note  only  in  so  far  as  it 


FIG.  112.— Transverse  section  through  the  embryo  of  a  Chick  on  the 
fourth  day  of  incubation  (about  100  times  the  natural  size).  The  primitive 
vertebra? "have  separated  into  the  outer  muscle-plate  (nip)  and  the  inner 
skeleton -pi  ate.  The  latter  below,  as  the  vertebral  bodies  (wh),  begins  to 
surround  the  notochord  (cTi)  ;  above,  as  the  vertebral  arches  (wV),  begins 
to  surround  the  medullary  tube  (m),  the  cavity  of  which  (mh),  is  already 
very  narrow.  At  wq  the  primitive  vertebra  passes  into  the  skin-muscle 
plate  of  the  ventral  wall  (lip)  ;  Jvpr,  leather-plate  of  the  dorsal  wall ;  h, 
horn-plate ;  a,  amm'on ;  ung,  primitive  kidney  duct ;  un,  primitive  urinary 
canal;  ao,  primitive  artery;  vc,  cardinal  vein ;  df,  intestinal-fibrous  layer; 
dd,  intestinal -glandular  layer;  dr,  intestinal  groove. 


THE  SKELETON  AND  MUSCLE  PLATES.        353 

indicates  an  internal  separation  of  the  primitive  vertebra 
into  two  entirely  distinct  parts  :  an  inner  part,  which  forms 
the  skeleton — the  skeleton-plate  (Fig.  95,  uw,  Fig.  112,  wb\ 
and  an  outer  part,  which  forms  the  muscle — the  muscle^ 
plate  (Figs.  95,  96,  m,  Fig.  112,  mp). 

The  skeleton-plate  is  formed  of  the  entire  inner  half  of 
each  primitive  vertebra,  immediately  adjoining  the  medul- 
lary tube  (Fig.  112,  wh,  wb).  Its  lower  part,  the  inner 
lower  corner  of  the  cube-shaped  primitive  vertebra,  splits 
up  into  two  lamellae,  which  grow  round  the  chord,  thus 
forming  the  basis  of  the  vertebral  bodies  (wli).  The 
upper  lamella  forces  its  way  between  the  chorda  and  the 
medullary  tube,  the  lower  lamella  between  the  chorda  and 
the  intestinal  tube  (Figs.  68,  69,  p.  276 ;  Fig.  93).  As  the 
lamellae  of  two  opposite  primitive  vertebral  pieces  come 
together  from  right  and  left  and  unite,  a  ring-like  sheath 
is  formed  round  that  particular  part  of  the  notochord. 
From  this  afterwards  arises  a  vertebral  body,  i.e.,  the 
massive,  lower,  or  ventral  portion  of  the  bony  ring,  which, 
as  a  vertebra  in  the  strict  sense,  surrounds  the  medullary 
tube  (Figs.  113-115).  The  upper  or  dorsal  half  of  this 
bony  ring,  the  vertebral  arch  (Fig.  112,  wb)  arises  in  just 
the  same  way  from  the  upper  portion  of  the  skeleton-plate ; 
i.e.,  from  the  inner,  upper  corner  of  the  cube-shaped  primi- 
tive vertebra.  The  two  inner,  upper  corners  of  two  oppo- 
site primitive  vertebras  coalesce,  from  right  to  left,  over  the 
medullary  tube,  resulting  in  the  closing  of  the  vertebral 
arch.  Between  each  pair  of  vertebral  arches  appear,  at  a 
later  period,  the  roots  of  the  spinal  nerves,  which  arise  from 
the  same  portion  of  the  skeleton-plate  (Fig.  98,  g,  -y,  p.  318). 

The  whole  secondary  vertebra,  which  thus  results  from 


354  THE   EVOLUTION    OF   MAN. 

the  coalescence  of  the  skeleton-plates  of  a  pair  of  primitive 
vertebrae,  and  which  encloses  within  itself  a  part  of  the 
chorda,  consists  originally  of  a  somewhat  soft  cell-mass,  which 
afterwards  passes  into  a  second  firmer,  cartilaginous  state, 
and  finally  into  a  third,  permanent,  bony  state.  These  three 
different  conditions  are  generally  distinguishable  in  the 
greater  part  of  the  skeleton  of  the  higher  Vertebrates ;  at 


PIG.  113  — Third  human  neck-vertebra. 
FIG.  114. — Sixth  human  chest-vertebra. 
FIG.  115. — Second  human  lumbar-vertebra. 

first,  most  parts  of  the  skeleton  are  quite  tender,  soft,  and 
membranous;  then,  in  the  course  of  development,  they 
become  cartilaginous,  and  finally  they  ossify. 

All  the  bony  vertebrae  which  afterwards  compose  the 
backbone,  or  vertebral  column,  arise,  as  we  have  already 
observed,  entirely  from  the  inner  portion  of  the  primitive 
vertebrae,  from  the  skeleton-plate.  The  outer  portion,  on 
the  other  hand,  which  we  have  called  the  "  muscle-plate  " 
(Fig.  112,  mp),  produces  the  great  mass  of  the  dorsal 
muscles  (the  dorsal  "  side  muscles  of  the  trunk  "),  as  well  as 
the  leather  skin,  which  covers  the  flesh  of  the  back.  This 
muscle-plate  is  in  direct  communication  with  that  portion 
of  the  side-plates  which  develops  into  the  ventral  skin  and 
the  ventral  muscles. 


DEVELOPMENT  OF  THE  SKULL.  355 

In  front,  at  the  head  end  of  the  embryo,  the  middle 
layer  (mesoderma)  does  not  split  into  primitive  vertebrae  and 
side-plates,  and  the  original  fibrous  layer  here  remains  un- 
divided, forming  the  so-called  "  head-plates  "  (Fig.  102,  k,  p. 
337).  From  these  arise  the  skull — the  bony  covering  of  the 
brain — as  well  as  the  muscles  and  leather-skin  of  the  head. 
The  skull  develops  precisely  in  the  same  way  as  the  mem- 
branous vertebral  column.  The  right  and  left  head-plates 
arch  towards  each  other  over  the  brain-bladder,  enclose  the 
anterior  extremity  of  the  chorda,  and  thus  eventually  form 
a  simple  soft,  membranous  capsule  round  the  brain.  This 
afterwards  changes  into  a  cartilaginous  primitive  skull, 
similar  to  that  which  is  retained  throughout  life  by  many 
fishes.  It  is  only  much  later  that  the  permanent  bony 
skull,  with  all  its  different  parts,  is  formed  from  this  cartila- 
ginous primitive  skull. 

In  the  embryo  of  Man,  as  in  that  of  all  other  Vertebrates, 
the  very  remarkable  and  important  structures,  which  are 
called  the  gill-arches  and  gill-openings,  appear,  at  a  very 
early  period,  on  each  side  of  the  head  (Plate  I.  Fig.  1,  and 
Figs.  116,  118,  /).  These  are  among  the  characteristic  and 
never-failing  organs  of  the  Vertebrates,  for  which  reason 
we  mentioned  them  in  considering  the  typical  primitive 
Vertebrate  (Figs.  52,  53,  p.  256).  On  the  right  and  left  wal  Is 
of  the  intestinal  head-cavity,  in  the  anterior  portion,  first 
one,  and  then  several  pairs  of  sac-like  protuberances  are 
formed,  which  break  through  the  entire  thickness  of  the 
side  wall  of  the  head.  They  thus  become  slits  through 
which  there  is  a  free  passage  from  without  into  the  throat- 
cavity:  these  are  the  gill-openings,  or  throat-openings 
Between  each  pair  of  gill-openings  the  wall  of  the  throat 


356 


THE   EVOLUTION   OF   MAN. 


cavity  grows  thicker,  and  is  changed  into  a  bow-shaped  or 
sickle-shaped  ridge :  these  are  the  gill-arches ;  on  their 
inner  side  a  vascular  arch  afterwards  arises  (Fig.  101,  p.  336). 


6 


FIGS.  116, 117.—  Head  of  a  Chick,  on  the  third  day  of  incubation :  116  is  a 
front  view ;  117  from  the  right  side  ;  n,  nose-rudiments  ;  I,  eye-rudiments  ; 
g,  ear-rndiments ;  v,  front-brain  ;  gl,  eye-fissures.  The  first  of  the  three 
pairs  of  gill-openings  is  separated  into  an  upper  jaw  process  (c)  and  a  lower 
jaw  process  (u).  (After  Kdlliker.) 

Fig.  118. — Head  of  embryo  of  a  dog,  from  the  front :  a,  the  two  side  halves 
of  the  front  brain-bladder ;  b,  eye-rudiments;  c,  middle  brain-bladder ;  de,  the 
first  pair  of  gill-arches  (c,  upper  jaw  process ;  d,  lower  jaw  process)  ;  /,  /',  /", 
the  second,  third,  and  fourth  pair  of  gill-arches ;  g  h  i  k,  heart  (g,  right,  h, 
left  auricle ;  i,  left,  k,  right  ventricle) ;  I,  origin  of  the  aorta,  with  three 
pairs  of  aorta-arches,  which  pass  on  to  the  gill-arches.  (After  Bischoff.) 

The  number  of  the  gill-arches,  and  of  the  gill-openings, 
which  alternate  with  the  former,  amounts  in  the  higher  Ver- 
tebrates to  four  or  five  on  each  side  (Fig.  118,  e,  d,  f,  f,  /'). 
The  lower  Vertebrates  have  a  yet  larger  number.  Origin- 
ally these  remarkable  formations  discharged  the  function  of 
a  breathing-apparatus — were  gills.  Even  yet  in  the  Fishes 
generally,  water,  serving  for  respiration,  and  which  is  taken 
in  through  the  mouth,  passes  out  through  the  gill  slits  on 


GILL-ARCHES  AND   GILL-OPENINGS.  357 

the  side  of  the  gullet.  In  the  higher  Vertebrates  they  after- 
wards close.  The  gill-arches  are  transformed  partly  into  the 
jaws,  partly  into  the  tongue-bone  and  the  bonelets  of  the 
ear  (ossicula  auditus).  (Of.  Plates  I.,  VI.,  and  VII.) 

Almost  simultaneously  with  the  development  of  the  gill- 
arches,  and  immediately  behind  these,  the  heart  with  its  four 
compartments  is  formed  (Fig.  11 8,0  h  i  &),  and  above,  on  the 
sides  of  the  head,  the  rudiments  of  the  higher  sense-organs 
appear  :  iio.se,  eye,  and  ear.  These  highly  important  organs 


FtG.  119. — Transverse  section  through  the  shoulder  region  and  the  front 
limbs  (wing-rudiments)  of  a  Chick,  on  the  fourth  day  of  incubation  (about 
20  times  the  natural  size).  Near  the  intestinal  tube  three  lighter  cords  are 
visible  on  each  side  in  the  dark  dorsal  wall,  which  pass  into  the  rudiments  of 
the  fore  limbs  or  wings  (e).  The  upper  of  these  is  the  muscle-plate,  the 
middle  is  the  hind,  and  the  lower  is  the  front  root  of  a  spinal  nerve.  In 
the  middle,  below  the  chorda,  is  the  aorta,  and  on  each  side  of  this  a  cardinal 
vein ;  below  the  latter  are  the  primitive  kidneys.  The  intestine  is  almost 
closed.  The  ventral  wall  extends  into  the  amnion,  which  forms  a  closed 
covering  round  the  embryo.  (After  Remak.) 


3$8  THE   EVOLUTION   OF   MAN. 

originate  in  the  very  simplest  form.  The  organ  of  smell, 
or  the  nose,  appears  quite  in  the  front  of  the  head,  in  the 
shape  of  two  little  pits  above  the  mouth-opening  (Fig. 
117,  n).  The  organ  of  sight,  or  eye,  also  in  the  form  of 
a  pit  (Fig.  117,  I,  118,  6),  comes  next,  behind  the  organ  of 
smell,  towards  which  a  considerable  vesicular  outgrowth  of 
the  fore-brain  grows  on  both  sides  of  the  head  (Fig.  105,  a). 
Further  back  appears  a  third  pit  on  each  side  of  the  head, 
the  first  rudiment  of  the  organ  of  hearing  (Fig.  117,  <?). 
No  trace  of  the  very  marvellous  future  structure  of  these 
organs,  or  of  the  characteristic  form  of  the  face,  is  yet  to 
be  seen. 

The  human  embryo,  having  reached  this  stage  of  develop- 
ment, is  yet  hardly  distinguishable  from  the  germ  of  any 
of  the  higher  Vertebrates.  (Of.  Plates  I,  VI.,  and  VII.)  All 
the  essential  portions  of  the  body  are  now  begun  :  the  head 
with  its  primitive  skull,  the  rudiments  of  the  three  higher 
sense-organs,  and  the  five  brain-bladders,  and  the  gill-arches 
and  gill-openings ;  the  trunk  with  the  medulla,  the  rudi- 
ments of  the  vertebral  column,  the  chain  of  metamera,  the 
heart  and  principal  blood-vessels,  and,  finally,  the  primitive 
kidneys.  Man,  in  this  germ-stage,  is  a  higher  Vertebrate,  and 
yet  there  is  no  essential,  morphological  difference  between 
the  human  embryo  and  that  of  Mammals,  Birds,  Reptiles, 
etc.  (Plates  VI.  and  VII.,  upper  line  of  sections).  This  is  an 
ontogenetic  fact  of  the  highest  significance;  from  it  are 
drawn  the  most  important  phylogenetic  conclusions. 

There  is,  however,  as  yet  no  trace  of  limbs.  Though 
the  head  and  the  trunk  are  already  separated,  though  all 
the  important  inner  organs  are  begun,  there  is  as  yet  no 
trace  of  the  limbs,  or  extremities,  in  this  stage.  These  do 


ORIGIN   OF   THE   LIMBS. 


359 


not  appear  till  later.  This  also  is  a  fact  of  the  profoundest 
interest;  for  it  tells  us  that  the  older  Vertebrates  were 
footless,  as  the  lowest  living  Vertebrates  (Arnphioxus  and 


FIG.  120. — Transverse  section  through  the  pelvic  region  and  the  hind 
limbs  of  a  Chick,  on  the  fourth  day  of  incubation  (about  40  times  the  natura 
size)  :  ft,  horn-plate  ;  w,  medullary  tube  ;  n,  spinal  canal ;  u,  primitive  kid- 
neys ;  x,  chorda ;  e,  hind  limbs ;  b,  allantois  canal  in  the  ventral  wall ;  t, 
aorta ;  v,  cardinal  veins ;  a,  intestine ;  d,  intestinal-glandular  layer ;  /,  in- 
testinal-fibrous layer ;  g,  germ-epithelium  ;  r,  dorsal  muscles  ;  c,  body-cavity 
(cceloma).  (After  Waldeyer.) 

the  Cyclostoma)  are  at  the  present  time.  The  descendants 
of  these  primaeval,  footless  Vertebrates  did  not  acquire  limbs 
till  a  much  later  period  in  the  course  of  their  development, 
when  they  acquired  four  limbs — a  front  pair  and  a  hind  pair. 
These  limbs  are  all  originally  formed  after  one  model,  though 
they  afterwards  develop  very  different!}'  :  in  Fishes  they 


360  THE   EVOLUTION   OF   MAN. 

become  fins  (pectoral  and  ventral) ;  in  Birds,  wings  and  legs  ; 
in  creeping  animals,  fore  and  hind  legs ;  in  Apes  and  in  Man, 
arms  and  legs.  All  these  parts  arise  from  a  first  rudiment  of 
the  same  perfectly  simple  form,  which  grows  secondarily  from 
the  skin-layer  (Figs.  119,  120).  They  always  make  their 
appearance  in  the  form  of  two  small  buds,  which  at  first 
resemble  mere  round  knobs  or  plates.  Gradually  each  of  these 
plates  increases  into  a  more  considerable  projection,  in  which 
there  is  a  more  slender  part,  nearer  the  body  of  the  embryo 
and  distinct  from  the  outer,  broader,  thicker  part.  This  later 
portion  is  the  rudiment  of  foot  or  of  hand,  while  the  former 
is  the  rudiment  of  arm  or  of  leg.  Plates  VI  and  VII.  show 
how  similar  are  the  rudimentary  limbs  in  very  different 
Vertebrates. 

A  careful  study  and  thoughtful  comparison  of  the 
embryos  of  Man  and  other  Vertebrates  in  this  stage  of 
development  is  very  instructive,  and  reveals  to  the  thought- 
ful many  profounder  mysteries  and  weightier  truths  than 
are  to  be  found  in  the  so-called  "revelations"  of  all  the 
ecclesiastical  religions  of  the  earth.  Compare,  for  instance, 
carefully  and  attentively  the  three  consecutive  stages  of 
development,  represented  in  Plate  VI.  of  the  Fish  (F),  Sala- 
mander (S),  Tortoise  (T),  and  Chick  (0),  and  in  Plate  VII.  the 
corresponding  embryos  of  the  Hog  (#),  Calf  (C),  Ra,bbit  (E), 
and  of  Man  (M}.  In  the  first  stage  (upper  row  of  Section 
I.),  in  which  the  head  with  the  five  brain-bladders,  and 
the  gill-arches  are  indeed  begun,  though  the  limbs  are  still 
entirely  wanting,  the  embryos  of  all  Vertebrates  from  Fish 
to  Man  differ  not  at  all,  or  only  in  non-essential  points.  In 
the  second  stage  (middle  row  of  Section  II.),  in  which  the 
limbs  arc  indicated,  differences  begin  to  appear  between  the 


THREE   STAGES   IN    DEVELOPMENT.  36! 

embryos  of  the  lower  and  the  higher  Vertebrates ;  as  yet, 
however,  the  embryo  of  Man  is  hardly  distinguishable  from 
that  of  the  higher  Mammals.  Finally,  in  the  third  stage 
(lower  row  of  Section  III.),  in  which  the  gill-arches  have 
already  disappeared  and  the  face  is  formed,  the  differences 
become  more  evident,  and  grow,  henceforth,  more  and  more 
striking.  The  significance  of  such  facts  as  these  cannot  be 
over-estimated.100 

If  there  exists  an  inner,  causal  connection  between  the 
incidents  of  germ-history  and  those  of  tribe-history,  as  in 
accordance  with  the  law  of  heredity,  we  must  assume  then 
these  ontogenetic  facts  immediately  afford  most  important 
phylogenetic  conclusions.  For  the  wonderful  and  compre- 
hensive harmony  between  the  individual  evolution  of  Man 
and  that  of  other  Vertebrates  is  only  explicable  by  assuming 
the  descent  of  these  from  a  common  parent-form.  Indeed 
this  common  descent  is  now  granted  by  all  able  naturalists; 
who  in  place  of  a  supernatural  creation  assume  a  non- 
miraculous  evolution  of  organisms. 


362  THE   EVOLUTION   OF   MAN. 


EXPLANATION  OF  PLATES  VI.  AND  VII. 

Plates  VI.  and  VII.  are  meant  to  represent  the  more  or  less  complete 
agreement,  as  regards  the  most  important  relations  of  form,  between  the 
embryo  of  Man  and  that  of  other  Vertebrates  in  early  stages  of  individual 
development.  This  agreement  is  the  more  complete,  the  earlier  the  period 
at  which  the  human  embryo  is  compared  with  those  of  other  Vertebrates. 
It  is  retained  longer,  the  more  nearly  related  in  descent  the  respective 
matured  animals  are — corresponding  to  the  "  law  of  the  ontogenetio  con- 
nection of  systematically  related  forms."  (Cf.  Chapter  XII.,  p.  366.) 

Plate  VI.  represents  the  embryos  of  two  of  the  lower,  and  two  of  the 
higher  Vertebrates  in  three  different  stages  :  of  a  Pish  (Osseous-fish,  F) ;  of 
an  Amphibian  (Land-salamander,  S) ;  of  a  Reptile  (Tortoise,  T)  ;  and  of  a 
Bird  (Chick,  C). 

Plate  VIII.  shows  the  embryos  of  four  Mammals  in  the  three  correspond- 
ing stages  :  of  a  Hog  (if),  Calf  (0),  Rabbit  (fi),  and  a  Man  (M).  The  con- 
ditions of  the  three  different  stages  of  development,  which  the  three  cross- 
rows  (I.,  II.,  III.)  represent,  are  selected  to  correspond  as  exactly  as  possible. 

The  first,  or  upper  cross-row,  I.,  represents  a  very  early  stage,  with  gill- 
openings,  and  without  limbs.  The  second  (middle)  cross-row,  II.,  shows  a 
somewhat  later  stage,  with  the  first  rudiments  of  limbs,  while  the  gill- 
openings  are  yet  retained.  The  third  (lowest)  cross-row,  III.,  shows  a  still 
later  stage,  with  the  limbs  more  developed  and  the  gill-openiugs  lost.  The 
membranes  and  appendages  of  the  embryonic  body  (the  amnion,  yelk-sac, 
allantois)  are  omitted.  The  whole  twenty -four  figures  are  slightly  magnified, 
the  upper  ones  more  than  the  lower.  To  facilitate  the  comparison,  they  are  all 
reduced  to  nearly  the  same  size  in  the  cuts.  All  the  embryos  are  seen  from 
the  left  side ;  the  head  extremity  is  above,  the  tail  extremity  below ;  the 
arched  back  turned  to  the  right.  The  letters  indicate  the  same  parts,  in 
all  the  twenty -four  figures,  namely  :  v,  fore-brain ;  z,  twixt-brain  ;  m,  mid- 
brain  ;  h,  hind-brain ;  n,  after-brain;  r,  spinal  marrow;  e,  nose;  a,  eyej 
o,  ear ;  k,  gill-arches ;  g,  heart }  w,  vertebral  column  j  /,  fore-liinbs ;  b,  hind- 
limbs ; 


HABCKEL'S  HVOLUTION  OF  MAN. 


RABCKBL'S  RVOLUTIOM  OP  MAN. 


PLATE  I'll 


CHAPTER  XII. 

THE   GEKM-MEMBRANES   AND   THB   FIRST   CIRCULATION 
OP    THE   BLOOD. 

The  Mammalian  Organization  of  Man. — Man  has  the  same  Bodily  Structure 
as  all  other  Mammals,  and  his  Embryo  develops  in  exactly  the  same 
way. — In  its  Later  Stages  the  Human  Embryo  is  not  essentially 
different  from  those  of  the  Higher  Mammals,  and  in  its  Earlier  Stages 
not  even  from  those  of  all  Higher  Vertebrates. — The  Law  of  the 
Ontogenetic  Connection  of  Systematically  Related  Forms. — Application 
of  this  Law  to  Man. — Form  and  Size  of  the  Human  Embryo  in  the 
First  Four  Weeks. — The  Human  Embryo  in  the  First  Month  of  its 
Development  is  formed  exactly  like  that  of  any  other  Mammal.— In  the 
Sacond  Month  the  First  Noticeable  Differences  appear. — At  first,  the 
Human  Embryo  resembles  those  of  all  other  Mammals;  later,  it 
resembles  only  those  of  the  Higher  Mammals. — The  Appendages  and 
Membranes  of  the  Human  Embryo. — The  Yelk-sac. — The  Allantois  and 
the  Placenta.— The  Amnion.— The  Heart,  the  First  Blood-vessels, 
and  the  First  Blood,  arise  from  the  Intestinal-fibrous  Layer. — The 
Heart  separates  itself  from  the  Wall  of  the  Anterior  Intestine. — The 
First  Circulation  of  the  Blood  in  the  Germ-area  (a.  germinativa)  : 
Yelk-arteries  and  Yelk-veins. — Second  Embryonic  Circulation  of  the 
Blood,  in  the  Allantois :  Navel-arteries  and  Navel-veins. — Divisions  of 
Human  Germ-history. 

"Is  man  a  peculiar  organism?  Does  he  originate  in  a  wholly  different 
way  from  a  dog,  bird,  frog,  or  fish  ?  and  does  he  thereby  justify  those  who 
assert  that  he  has  no  place  in  nature,  and  no  real  relationship  with  the 
lower  world  of  animal  life  ?  Or  does  he  develop  from  a  similar  embryo, 
and  undergo  the  same  slow  and  gradual  progressive  modifications  ?  The 
answer  is  not  for  an  instant  doubtful,  and  has  not  been  doubtful  for  the  last 
thirty  years.  The  mode  of  man's  origin  and  the  earlier  stages  of  hia 
20 


364  THE   EVOLUTION   OF   MAN. 

development;  are  undoubtedly  identical  with  those  of  the  animals  standing 
directly  below  him  in  the  scale ;  without  the  slightest  doubt,  he  stands  in 
this  respect  nearer  the  ape  than  the  ape  does  to  the  dog." — THOMAS  HUXLEY 
(1863). 

THE  most  important  phenomenon,  having  a  general  bearing, 
that  we  have  so  far  met  with  in  the  process  of  human  germ- 
history,  is  surely  the  fact  that  the  development  of  the 
human  body  proceeds  from  the  beginning  in  exactly  the 
same  way  as  that  of  other  Mammals.  All  the  special 
peculiarities  of  individual  development  which  distinguish 
Mammals  from  all  other  animals  are  found  also  in  Man. 
Long  ago,  from  the  physical  structure  of  the  perfect  Man 
the  conclusion  was  drawn  that  his  natural  position  in  the 
system  of  the  animal  world  can  only  be  in  the  mammalian 
class.  In  1735  Linnaeus,  in  his  Sy  sterna,  Naturae,  placed 
Man  in  one  and  the  same  class  with  the  Apes.  This  position 
is  fully  corroborated  by  comparative  germ-history.  We 
have  evidence  that,  no  less  in  embryonic  development  than 
in  anatomical  structure,  Man  closely  resembles  the  higher 
Mammals,  and  especially  the  Apes.  If  we  now  seek,  by 
applying  the  fundamental  biogenetic  law,  to  understand  this 
ontogenetic  agreement,  the  perfectly  simple  and  necessary 
conclusion  is  that  Man  is  descended  from  other  mammalian 
forms.  Hence  we  can  no  longer  doubt  the  common  descent 
of  Man  and  the  other  Mammals  from  a  single  primaeval 
parent-form,  or  hesitate  to  believe  that  the  blood-relation- 
ship is  closest  between  Men  and  Apes. 

This  essential  harmony  between  the  embryo  of  Man 
and  of  the  other  Mammals,  in  their  whole  bodily  form  and 
internal  structure,  exists  even  in  that  latest  age  of  develop- 
ment, in  which  the  mammalian  body,  as  such,  is  already 


HOMOLOGY  BETWEEN  HUMAN  AND  MAMMALIAN  GERMS.  365 

unmistakable.  (Cf.  Plates  VI.  and  VII.,  the  second  row.) 
But  in  a  somewhat  earlier  stage,  in  which  the  rudi- 
ments of  the  limbs,  the  gill-arches,  the  sense-organs,  etc., 
are  already  present,  we  cannot  yet  recognize  mammalian 
embryos  as  such,  nor  can  we  distinguish  them  from  the 
embryos  of  Birds  and  Reptiles.  Jf  we  go  back  to  still  earlier 
stages  of  development,  we  are  unable  even  to  discover  any 
distinction  between  the  embryos  of  these  higher  Vertebrates 
and  those  of  the  lower,  such  as  the  Amphibia  and  Fishes 
(Plates  VI,  VII.,  upper  row).  Finally,  if  we  go  still  further 
back,  to  the  construction  of  the  body  from  the  four 
secondary  germ-layers,  we  make  the  surprising  discovery 
that  these  same  four  germ-layers  exist,  not  only  in  all 
Vertebrates,  but  also  in  all  the  higher  Invertebrates,  and 
that  they  are  everywhere  concerned  in  the  same  way  in 
forming  the  fundamental  organs  of  the  body.  And  if  then 
we  inquire  into  the  origin  of  these  four  secondary  germ- 
layers,  we  find  that  they  develop  from  the  two  primary 
germ-layers,  which  are  identical  in  all  animals,  with  the 
exception  of  the  lowest  division,  the  Protista.  (Cf.  Figs. 
23-28,  p.  93.)  Finally,  we  see  that  the  cells,  which  compose 
the  two  primary  germ-layers,  universally  originate  by 
fission,  from  a  single  simple  cell,  from  the  egg-cell. 

It  is  impossible  to  lay  too  much  stress  on  this  remark- 
able parallelism  of  the  most  important  germ-conditions  of 
man  and  animals.  We  shall  afterwards  turn  the  fact  to 
account  in  support  of  the  hypothesis  of  monophyletic 
descent,  i.e.,  the  assumption  of  the  common,  single  line  of 
descent  of  man  and  the  higher  animal  tribes.  It  declares 
itself  in  the  very  beginning  of  the  individual  development ; 
in  the  cleavage  of  the  egg-cell,  in  the  formation  of  the 


366  THE   EVOLUTION    OF   MAN, 

germ-layers,  in  the  fission  of  these,  in  the  construction  of  the 
most  .important  fundamental  organs  from  these  germ-layers, 
etc.  The  first  rudiments  of  the  principal  parts  of  the  body, 
and,  above  all,  of  the  oldest  main  organ,  the  intestinal  canal, 
are  everywhere  originally  identical ;  they  always  appear  in 
the  same  simplest  form.  But  all  the  peculiarities  by  which 
the  various  larger  and  smaller  groups  of  the  animal  kingdom 
are  differentiated  from  one  another  only  make  their  appear- 
ance gradually,  and  secondarily,  in  the  course  of  the  evolu- 
tion of  the  germ ;  and  those  which  distinguish  the  animals 
most  closely  allied  in  the  system  of  the  animal  kingdom 
are  the  latest  to  appear.  This  latter  phenomenon  can  be 
formulated  as  a  definite  law,  which  may  be  regarded  as,  in 
some  sense,  an  addition  or  appendage  to  the  fundamental 
law  of  Biogeny.  It  is  the  law  of  the  ontogenetic  connection 
between  systematically  allied  'animal  forms.  The  meaning 
of  this  is  that  the  nearer  two  full-grown  perfect  animals  are 
to  each  other  in  point  of  general  body-structure,  and  hence 
the  more  closely  they  are  allied  in  the  system  of  the  animal 
kingdom,  the  longer  do  their  embryonic  forms  remain  the 
same,  and  the  longer  are  their  embryos,  and  their  young 
forms  in  general,  either  altogether  indistinguishable,  or  dis- 
tinguishable only  by  subordinate  characters.  This  law 
holds  good  of  all  animals  in  which  the  original  form  of  evo- 
lution has  been  correctly  inherited  palin genetically,  or  by 
"  inherited  evolution  ".  Where,  on  the  other  hand,  this  ori- 
ginal form  has  been  altered  kenogenetically,  or  by  "  vitiated 
evolution,"  the  law  is  less  true  in  proportion  as  a  greater 
number  of  new  evolutionary  conditions  have  been  intro- 
duced by  adaptation  (cf.  pp.  10-1 4).101 

If  we    apply  this   law   of  the   ontogenetic  connection 


ONTOGENETIC   RELATION   OF   ALLIED   FORMS.  367 

between  systematically  (and  hence  also  phylogenetically) 
allied  forms  to  Man,  and  if,  with  reference  to  this  law,  we 
rapidly  run  through  the  earliest  human  conditions,  the  first 
striking  thing  noticeable  in  the  early  history  of  the  germ 
is  the  morphological  identity  of  the  egg-cells  of  Man  and 
of  other  Mammals  (Fig.  1).  All  the  properties  that  cha- 
racterize the  mammalian  egg,  are  also  observable  in  the 
human  egg;  especially  that  characteristic  structure  of  its 
coating  (the  zona  pellucida)  which  clearly  distinguishes 
the  mammalian  egg  from  that  of  all  other  animals.  When 

FIG.  121. — Lyre-shaped  germ-shield  of  a  dog. 
"  Double  shield  "  of  Remak,  "  embryonic  rudiment  ' 
of  other  authors.)  The  dorsal  furrow  is  visible  in  the 
centre  ;  on  eithersi.de  are  the  medullary  swellings. 

the  human  embryo  is  fourteen  days  old, 
it,  like  all  other  mammalian  embryos,  is 
in  the  form  of  an  entirely  simple,  lyre- 
shaped  germ-shield.  Along  the  middle 
line  of  the  dorsal  side  of  this,  there  ap- 
pears the  rectilineal,  groove-shaped  medul- 
lary furrow,  bordered  by  the  two  parallel 
dorsal,  or  medullary  swellings.  The  ventral  side  is  attached 
to  the  wall  of  the  globular  intestinal  germ- vesicle.  In  this 
stage  the  human  embryo  is  one  line,  or  two  millimetres 
in  length.  It  is  not  distinguishable  from  that  of  other 
Mammals,  e.g.,  of  the  Dog  (Fig.  121).10'2 

A  week  later,  or  at  the  end  of  the  twenty-first  day,  the 
human  embryo  has  already  attained  twice  this  length  :  it  is 
now  two  lines  or  about  five  millimetres  in  length  and  already 
shows,  when  seen  from  the  side,  the  characteristic  curvature 
of  the  back,  the  swelling  of  the  head  end,  the  earliest  rudi- 


THE    EVOLUTION    OF    MAN. 


368 

ments  of  the  higher  sense-organs,  and  the  rudiments  of  tho 
gill-openings,  piercing  the  sides  of  the  neck  (Fig.  122,  III. ; 
Plate  VII.  Fig.  M  I.).  The  allantois  has  growr  out  from  the 


FIG.  122. — Human  germs  or  embryos  from  the  second  to  the  fifteen!  h 
week  (natural  size),  seen  from  the  left  side,  the  arched  back  turned  towards 
the  right.  (Principally  after  Ecker.)  II.,  human  embryo  of  14  days ;  III.,  of 
3  weeks;  IV.,  of  4  weeks;  V.,of  5  weeks ;  VI.,  of  6  weeks;  VII.,  of  7  weeks: 
VIIL,  of  8  weeks  ;  XII.,  of  12  weeks ;  XV.,  of  15  weeks. 

hind  end  of  the  intestine.  The  embryo  is  already  entirely 
enveloped  by  the  amnion,  and  is  now  only  connected  with 
the  germ-vesicle,  which  is  changing  into  the  yelk-sac,  by 
means  of  the  yelk-duct,  in  the  centre  of  the  abdomen. 


DEVELOPMENT   OF   THE   HEAD.  360 

In  this  stage  of  development,  the  extremities,  or  limbs, 
are  still  entirely  wanting ;  there  is  as  yet  no  trace  either  of 
arms  or  legs.  The  head  end,  however,  has  already  become 
markedly  distinct  or  differentiated  from  the  tail  end ;  more- 
over, the  first  rudiments  of  the  brain-bladders  appear  in 
front,  and  the  heart  appears  more  or  less  distinctly  on  the 
anterior  intestine.  A  real  face  is,  however,  not  yet  formed. 
We  may  also  search  in  vain  for  any  character  distinguishing 
the  human  embryo,  in  this  stage,  from  that  of  other  Mammals. 
(Of.  Fig.  ML,RL,  01.,  and  HI.  on  Plate  VII.)103 

Another  week  later,  at  the  end  of  the  fourth  week, 
between  the  twenty-eighth  and  the  thirtieth  day  of  develop- 
ment, the  human  embryo  is  four  or  five  lines  in  length,  or 
about  one  centimetre  (Fig.  122,  IV. ,  Plate  VII.  Fig.  M  II.). 
The  head  with  its  various  parts  is  now  plainly  distinguish- 
able :  within,  the  five  primitive  brain-bladders  (fore-brain, 
mid-brain,  twixt-brain,  hind-brain,  and  after-brain) ;  at  the 
lower  end  of  the  head,  the  gill-arches,  which  divide  the 
gill-openings ;  on  the  sides  of  the  head  the  rudiments  of 
the  eyes,  two  indentations  of  the  outer  skin,  towards  which 
grow  two  simple  bladders  from  the  side-wall  of  the  fore- 
brain.  Far  behind  the  eyes,  above  the  last  gill-arch,  the 
bladder-like  rudiment  of  the  organ  of  hearing  is  visible. 
The  head,  which  is  very  large,  is  attached  to  the  trunk  at 
a  very  considerable  angle,  almost  a  right  angle.  The  trunk 
itself  is  still  attached  at  the  centre  of  its  ventral  side  to  the 
intestinal  germ-vesicle;  but  the  embryo  is  already  still 
further  separated  from  the  latter,  which,  therefore,  protrudes 
and  forms  the  yelk-sac.  Like  the  front  part,  the  hind  part 
of  the  body  is  very  much  curved,  so  that  the  pointed  tail 
end  is  turned  towards  the  head.  The  head  rests,  face  down- 


370 


THE   EVOLUTION    OF   MAN. 


FK;.  123. — Human  embryo  of  four  weeks  old,  opened  on  the  ventral  side. 
The  walls  of  the  chest  and  abdomen  have  been  cut  away,  so  that  the  contents 
of  the  chest  and  ventral  cavities  are  visible.  All  the  appendages  (amnion, 
allantois,  and  yelk-sac)  have  been  removed,  and  also  the  middle  portion  of 
the  intestine  :  n,  eye  ;  3,  nose  ;  4,  upper  jaw  ;  5,  lower  jaw  ;  6,  the  second 
pill -arch,  and  6"  the  third;  ov,  heart  (o,  right,  o',  left  auricle;  v,  right,  v', 
left  ventricle);  6,  origin  of  the  aorta;/,  liver  (u,  navel-vein);  e,  intestine 
(with  the  yelk-artery,  cut  away  at  a') ;  j',  yelk-vein  ;  m,  primitive  kidney  ; 
t,  rudiments  of  the  sexual  glands;  r,  terminal  intestine  (with  mesentery,  2,  cut 
away)  ;  n,  navel-artery ;  M,  navel-vein ;  7,  anus ;  8,  tail ;  9,  front  limb ;  9', 
hind  limb.  (After  Coste.) 


RUDIMENTS   OF   THE   L1MUS.  37! 

FIG,  124. — Human  embryo  of  five  weeks  old,  opened  on  the  ventral  side 
(as  in  Fig.  123).  The  chest  and  ventral  walls,  with  the  liver,  have  been 
removed ;  3,  outer  nasal  process  ;  4,  upper  jaw  ;  5,  lower  jaw  ;  z,  tongue  ;  v, 
right,  v',  left  ventricle  of  heart ;  o',  left  auricle ;  b,  origin  of  the  aorta ; 
6'  b"  b'",  first,  second,  third  arterial  arches ;  c,  c',  c",  hollow  veins  (vena 
cavce) ;  ae,  lungs  (y,  arteries  of  lungs) ;  e,  stomach ;  TO,  primitive  kidneys ; 
O',  left  yelk-veins;  s,  vena  portce ;  a,  right  yelk-artery  ;  n,  na vel -artery ; 
u,  navel-vein);  x,  yelk-duct;  i,  large  intestine;  8,  tail;  9,  front  limb; 
9',  hind  limb.  (After  Coste.) 

ward,  on  the  yet  open  chest.  The  curvature  presently 
becomes  so  great  that  the  tail  almost  touches  the  forehead 
(Fig.  122,  V. ;  Fig.  137).  Three  or  four  distinct  curves  of  the 
arched  dorsal  side  are  now  distinguishable ;  a  skull-curve  or 
"front  head-curve"  near  the  second  brain-bladder,  a  neck- 
curve  or  "  hind  head-curve  "  at  the  beginning  of  the  spinal 
marrow,  and  a  tail-curve  at  the  hind  end  of  the  body.  This 
marked  curvature  is  shared  by  Man  with  the  three 
higher  classes  of  Vertebrates  (the  Amnion-animals),  while 
in  the  lower  classes  it  is  either  much  less  pronounced,  or 
altogether  absent.  In  this  stage,  when  four  weeks  old,  Man 
has  a  true  tail,  double  the  length  of  the  legs.  The  rudi- 
ments of  the  limbs  are  now  plainly  marked:  four  entirely 
simple  buds  in  the  form  of  roundish  plates,  two  fore  limbs 
and  two  hind  limbs,  the  former  being  a  little  larger  than 
thelattei.104 

On  opening  the  human  embryo  of  the  age  of  one  month 
(Fig.  123),  we  find  the  intestinal  canal  already  formed  in  the 
body-cavity,  and  that  it  is  nearly  completely  separated 
from  the  germ-vesicle.  The  mouth-opening  and  anus 
already  exist.  The  cavity  of  the  mouth  is,  however,  not 
yet  separated  from  that  of  the  nose,  nor  is  the  face  in 
general  yet  formed.  The  heart,  on  the  other  hand,  already 
shows  all  the  four  compartments ;  it  is  very  large,  filling 


3/2  THE    EVOLUTION    OF   MAN. 

almost  the  entire  chest-cavity  (Fig.  123,  o-v).  Behind  it  the 
very  small  rudiments  of  the  lungs  lie  concealed.  The 
primitive  kidneys  are  very  large  (Fig.  123,  m),  occupying 
the  greater  part  of  the  ventral  cavity,  and  extending  from 
the  liver  (/)  to  the  pelvic  intestine.  Thus  at  the  end  of 
the  first  month,  all  the  essential  parts  of  the  bod}'  are 
already  begun;  and  yet,  in  this  stage,  we  are  still  unable 
to  discern  any  characters  essentially  distinguishing  the 
human  embryo  from  those  of  the  Dog,  the  Rabbit,  the 
Ox,  the  Horse,  or,  indeed,  of  any  of  the  higher  Mammals. 
All  these  embryos  are  still  of  the  same  form,  and  at  best 
differ  from  the  embryo  of  Man  only  in  the  general  dimen- 
sions of  the  body,  or  in  the  size  of  the  individual  organs — 
differences  of  no  moment.  Thus,  for  example,  the  head, 
relatively  to  the  trunk,  is  a  little  larger  in  Man  than  in  the 
Sheep ;  in  the  Dog  the  tail  is  somewhat  longer  than  in 
Man.  But  these  are  all,  evidently,  very  trifling  differences 
indeed,  and  of  no  importance.  On  the  other  hand,  the 
whole  internal  and  external  organization,  the  form,  the 
disposition,  and  the  connection  of  the  separate  parts  of  the 
body  of  the  germ  are  essentially  the  same  in  the  human 
embryo  of  four  weeks,  and  in  the  embryos  of  other 
Mammals  in  a  corresponding  stage  of  development. 

But  the  case  is  different  even  in  the  second  month  of 
human  development.  Fig.  122  represents  a  human  germ, 
VI.,  of  six  weeks,  VII.,  of  seven  weeks,  VIII.,  of  eight 
weeks,  in  the  natural  size.  The  differences  which  distin- 
guish the  human  embryo  from  those  of  the  Dog  and  the 
lower  Mammals,  now  gradually  begin  to  become  more 
prominent.  Even  after  the  sixth,  and  yet  more  after  the 
eighth  week,  considerable  differences  are  visible,  especially 


DEVELOPMENT  OF  THE  DISTINCTIVE  HUMAN  CHARACTERS.    373 

in  the  structure  of  the  head  (Plate  VII.  Fig.  M  III.,  etc.). 
The  size  of  the  various  divisions  of  the  brain  in  Man  is 
now  greater,  while,  on  the  contrary,  the  tail  appears  shorter. 
Other  differences  between  Man  and  the  lower  Mammals  are 
to  be  seen  in  the  relative  dimensions  of  the  interior  parts. 
Yet  even  now  the  human  embryo  is  hardly  distinguishable 
from  that  of  the  nearest  allied  Mammals,  the  Apes,  and 
especially  the  anthropomorphic  Apes.  The  characters  which 
distinguish  the  human  embryo  from  those  of  Apes  make  their 
appearance  much  later ;  even  in  a  very  advanced  stage  of 
development,  in  which  the  human  embryo  is  instantly 
distinguishable  from  that  of  hoofed  animals  (Ungulata),ihe 
former  is  still  very  similar  to  the  embryo  of  the  higher 
Apes.  At  length,  in  the  fourth  or  fifth  month  these  charac- 
ters make  their  appearance,  and  during  the  four  last  months 
of  the  embryonic  life  of  the  human  being,  from  the  sixth 
to  the  ninth  month  of  pregnancy,  the  human  embryo  is 
readily  distinguishable  from  those  of  all  other  Vertebrates ; 
then  the  characters  which  distinguish  the  various  races  of 
mankind  also  make  their  appearance,  .especially  those  in 
the  structure  of  the  skull. 

The  striking  resemblance  which  exists  for  a  long  time 
between  the  embryos  of  Man  and  of  the  higher  Apes  dis- 
appears, moreover,  at  a  much  earlier  period  in  the  lower 
Apes.  It  is  naturally  retained  longest  in  the  large  anthro- 
pomorphic Apes  (the  Gorilla,  Chimpanzee,  Orang-outang,  and 
Gibbon;  Plate  XIV.).  The  facial  resemblance,  which  strikes  us 
in  these  man-like  Apes,  continually  decreases  with  age.  On 
the  other  hand,  it  is  retained  throughout  life  by  the  remark- 
able Nose-apes  (Semnopithecus  nasicus)  of  Borneo  (Fig.  125), 
the  well-shaped  nose  of  which  might  well  be  coveted  by  men 


.3/4  THE   EVOLUTION   OF   MAN 

in  whom  this  organ  is  too  short.  On  comparing  the  face  of 
this  nosed  monkey  with  that  of  specially  ape-like  human 
beings  (e.g.,  the  noted  Julia  Pastrana,  Fig.  126),  the 


FIG.  125. — Head  of  a  nose-ape   (Semnopithecus  nasicus)   from  Borneo. 
(After  Brehm.) 

FIG.  126. — Head  of  Jnlia  Pastrana.     (From  a  photograph  by  Hintze.) 

former  will  appear  a  higher  form  of  development  than  the 
latter.  There  are  very  many  persons  who  believe  that  the 
"image  of  God"  is  unmistakably  reflected  in  their  own 
features.  If  the  Nosed-ape  shared  in  this  singular  opinion, 
he  would  hold  it  with  a  better  right  than  some  snub-nosed 
people.105 

This  gradually  progressive  separation,  this  increasing 
divergence  of  the  human  from  the  animal  form,  which 
depends  on  the  law  of  the  ontogentic  connection  between 
systematically  allied  forms,  is  seen  not  only  in  the  external 
structure  of  the  body,  but  also  in  the  formation  of  the 
internal  organs.  It  is  even  expressed  in  the  formation 
of  the  coverings  and  appendages  that  are  found  round  the 
outside  of  the  embryo,  and  which  we  are  now  about  to 
consider  somewhat  more  in  detail.  Two  of  these  appen- 
dages, the  amnion  and  the  allantois,  belong  only  to  the 


THE   HUMAN   EMBRYO.  375 

three  higher  vertebrate  classes,  while  the  third,  the  yelk- 
sac,  occurs  in  most  Vertebrates.  This  circumstance  is  very 
significant,  and  we  shall  afterwards  find  that  it  affords 
material  assistance  towards  the  construction  of  the 
genealogical  tree  of  Man. 

The  nature  of  the  outer  egg-membrane,  which  surrounds 
the  entire  egg  embedded  in  the  uterus  of  the  Mammal,  is 
the  same  in  Man  as  in  the  higher  Mammals.  At  first  the 
egg  is  surrounded,  as  we  have  already  stated,  by  the  trans- 
parent, structureless  zona  pellucida  (Fig.  1,  p.  122,  and  Fig. 
3G-40,  pp.  210-212).  Very  soon,  however,  even  in  the  first 
week  of  development,  its  place  is  taken  by  the  permanent 
tufted  membrane  (choriori).  This  originates  from  the  outer 
fold  of  the  amnion,  from  the  so-called  serous  membrane, 
the  formation  of  which  we  shall  presently  examine.  It  is 
Ibrmed  of  a  single  stratum  of  cells  from  the  outer  germ- 
iayer,  the  skin-sensory  layer  At  its  first  appearance  the 
serous  membrane  is  an  entirely  simple,  flat,  closed  vesicle , 
like  a  wide  sac,  closed  in  all  directions,  it  surrounds  the 
embryo  with  its  appendages;  the  intermediate  space  be- 
tween the  two  is  filled  with  clear  watery  fluid.  At  an 
early  period,  however,  the  smooth  outer  surface  of  the  sac 
becomes  covered  with  numerous  small  tufts  or  knots,  which 
are  really  hollow  processes,  resembling  the  fingers  of  a  glove 
(Fig.  127;  139,  4  sz,  5  chz).  These  branch  and  grow  into 
the  corresponding  depressions  formed  by  the  bag-like  glands 
of  the  mucous  membrane  of  the  maternal  uterus ;  the  egg 
thus  acquires  its  permanent,  fixed  position  (Figs.  130,  132, 
134). 

In  the  human  egg,  even  between  the  thirteenth  and 
fourteenth  day,  this  outer  egg-membrane,  which  we  shall 


3/6  THE    EVOLUTION   OF   MAN. 

a 


FIG.  127. 


FIG.  128. 


FIG.  130. 


FIG.  131. 


FIG.  127.— Human  egg  between  the  twelfth  and  thirteenth  day.  After 
Allen  Thomson.  1.  Not  opened ;  natural  size.  2.  Opened,  and  enlarged. 
Within  the  outer  tufted  membrane  (c/ionon)  the  small  curved  germ  lies  upon 
the  left  of  the  upper  side  'of  the  large  intestinal  germ-vesicle. 

FIG.  128. —  Human  egg  on  the  fifteenth  day.  After  Allen  Thomson. 
Natural  size,  and  opened.  The  small  germ  lies  in  the  upper  right-hand  part 
of  the  right  half. 

FIG.  129. — Human  germ  on  the  fifteenth  day,  taken  from  the  egg; 
enlarged  :  o,  yelk-sac  ;  b,  region  of  the  neck  (where  the  medullary  fuiTow  is 
already  closed) ;  c,  head  part  (with  open  medullary  furrow)  ;  d,  hind  part 
(with  open  medullary  furrow)  ;  e,  a  shred  of  the  amnion. 

FIG.  130. — Human  egg  between  the  twentieth  and  twenty-second  day. 
After  Allen  Thomson.  Natural  size  ;  opened.  The  outer  tufted  membrane 
(chorion)  forms  a  capacious  vesicle,  to  the  inner  wall  of  which  the  small 
g3rm  (above,  on  the  right)  is  attached  by  a  short  navel-cord. 

FIG.  131. — Humaii  germ  between  the  twentieth  and  twenty-second  day, 
taken  out  of  the  egg;  enlarged  :  a,  amnion;  b,  yelk-sac  ;  c,  lower  jaw  process 
of  the  first  gill-arch;  d,  upper  jaw  process  of  the  same;  e,  second  gill-arch 
(behind  it  are  two  other  small  arches).  Three  gill-openings  are  very  plainly 
seen  ;  /,  rudiments  of  the  fore-limbs ;  g~,  ear-vesicle  ;  h,  eye  ;  i,  heart. 


THE   C'HORION.  377 

briefly  call  the  tufted  membrane  (chorion),  is  completely 
covered  with  small  knots  or  tufts,  and  forms  a  globe  or 
sphere  of  6-8  millimetres  in  diameter  (Figs.  127-129.)  In 
consequence  of  the  accumulation  of  a  large  mass  of  liquid 
in  the  inside,  the  tufted  membrane  (chorion)  continually 
increases  in  size,  so  that  the  embryo  occupies  only  a  small 
part  of  the  space  within  the  egg-bladder.  At  the  same  time 
the  tufts  on  the  chorion  increase  in  number  and  size,  and 


FIG.  132.— Human  embryo,  with  amnion  and  allantois,  in  the  third  week ; 
with  a  large  globular  yelk-sac  (below)  and  a  bladder-like  allantois  (right)  ; 
there  are  as  yet  no  limbs.  The  germ  and  its  appendages  are  surrounded  by 
the  tufted  membrane  (chorion). 

FIG.  133.— Human  embryo,  with  amnion  and  allantois,  in  the  fourth 
week.  (After  Krause.)  The  amnion  (w)  lies  pretty  close  to  the  body.  The 
greater  part  of  the  yelk-sac  (d)  has  been  torn  away.  Behind  this  t 
allantois  (I)  is  visible,  as  a  pear-shaped  vesicle  of  considerable  size.  Arms 
(/)  and  legs  (6)  are  just  beginning;  v,  fore-brain;  z,  twixt-bram;  ro, 
mid-brain;  7*,  hind-brain;  n,  after-brain;  a,  eye;  k,  three  gill-arches;  c. 
heart ;  s,  tail. 


378  THE   EVOLUTION    OF    MAN. 

become  more  branched.  Though  these  tufts  at  first  covered 
the  whole  surface,  they  afterwards  degenerate  over  a  great 
part  of  this;  they  develop  in  consequence  all  the  more 
vigorously  at  a  particular  point,  at  the  place  where  the 
allantois  forms  the  placenta. 

On  opening  the  chorion  of  a  human  embryo  of  three 


FIG.  134. —  Human  embryo  with  its  membranes,  six  weeks  old.  The  outer 
covering  of  the  embryo  forms  the  chorion,  which  is  covered  witti  numerous 
branching  tufts,  and  is  lined  internally  by  the  serous  membrane.  The  embryo 
is  surrounded  by  the  delicate  membrane  of  the  amnion-sac.  The  yelk-sac  is 
reduced  to  a  little  pear-shaped  navel-vesicle  ;  its  thin  stalk,  the  long  yelk- 
duct,  is  enclosed  in  the  navel-cord.  In  this  cord,  behind  the  yelk-duct,  lies 
the  much  shorter  stalk  of  the  allantois,  the  inner  layer  of  which  (intestinal- 
glandular  layer)  in  Figs.  132  and  133  presented  a  bladder  of  considerable 
size ;  while  the  outer  layer  attaches  itself  to  the  inner  wall  of  the  outer 
egg -membrane,  and  at  this  point  forms  the  placenta. 


THE   YELK-SAC   AND   THE   PERMANENT   INTESTINE.      379 

weeks  old,  we  find  a  large,  round  sac,  filled  with  liquid, 
on  the  ventral  side  of  the  germ.     This  is  the  yelk-sac,  the 
so-called  navel-vesicle,  the  origin  of  which  we  have  already 
examined  (Figs.  132,  133).     In  proportion  as   the   embryo 
grows  larger,  the  yelk-sac  grows  smaller.     At  a  later  period 
it  hangs,  as  a  small  pear-shaped  vesicle,  at  the  end  of  a  long 
stalk  (the   yelk-duct),  from  the   abdomen  of  the   embryo 
(Fig.  139,  5  ds),  and  is  finally  detached  from  the  body  by  the 
closing  of  the  navel.    The  wall  of  this  navel-vesicle  consists, 
as  we  have  seen,  of  an  inner  layer,  the  intestinal-glandular 
layer,  and  an  outer  layer,  the  intestinal-fibrous  layer.     It  is 
therefore  composed  of  the  same  constituents  as  the  intestinal 
wall  itself,  of  which  it  forms,  in  fact,  a  direct  continuation. 
In  Birds  and  Reptiles  the  yelk-sac  is  much  larger,  and  con- 
tains a  considerable  quantity  of  albuminous  and  fatty  nutri- 
tive matter.    This  penetrates  through  the  yelk-duct  into  the 
intestinal  cavity  and  serves  as  food.     In  Mammals  the  yelk- 
sac  plays  a  much  smaller  part  in  the  nourishment  of  the 
germ,  and  degenerates  at  an  early  period.    The  relation  of 
the  intestine  to  the  yelk  sac  has  very  often  been  entirely 
mistaken     According  to  the  Gastrsea  Theory  the  two  form 
one  whole.     We  may  say  that  the  primitive  intestine  of 
those  Vertebrates  which   are   without   a  skull  afterwards 
separated    in    their   descendants    (in    consequence    of   the 
accumulation  of  nutritive  yelk)  into  two  part's,  a  transitory 
embryonic  organ  (the  yelk-sac),  and  a  permanent  intestine 
(the  after-intestine). 

Behind  the  yelk-sac,  a  second  and  much  more  significant 

appendage  forms,  at  an  early  period,  on  the  abdomen  of  the 

vertebrate   embryo.      This    is    the  allantois,   or  primitive 

urinary  sac,  an  important  embryonic  organ,  which  occurs 

27 


380 


THE  EVOLUTION   OF   MAN. 


only  in  the  three  higher  classes  of  Vertebrates.  It  grows 
from  the  hind  end  of  the  intestinal  canal,  from  the  pelvic 
intestinal  cavity  (Figs.  133, 1,  135,  r,  u,  136,  p,  139,  al).  Its 


FIG.  135. — Longitudinal  section  through  the  embryo  of  a  Chick  (in  the 
fifth  day  of  incubation).  The  embryo  with  curved  dorsal  surface  (black)  : 
d,  intestine  ;  o,  mouth ;  o,  anus ;  I,  lungs ;  h,  liver ;  g,  mesentery ;  v,  auricle ; 
fc,  ventricle ;  b,  arterial  arches ;  t,  aorta ;  c,  yelk-sac ;  m,  yelk-duct ;  w, 
allantois ;  r  stalk  of  allantois ;  n,  amnion  ;  w,  amnion- cavity ;  s,  serous 
membrane.  (After  Baer.) 

first  rudiment  appears  as  a  small  vesicle  on  the  edge  of  the 
pelvic  intestinal  cavity,  representing  an  extension  of  the 
intestine,  and  therefore  (like  the  yelk-sac)  has  a  two-layered 
wall.  The  cavity  of  the  vesicle  is  coated  by  the  intestinal- 
glandular  layer,  and  the  outer  lamella  of  the  wall  is  formed 
by  the  thickened  intestinal-fibrous  layer.  The  small  vesicle 
grows  larger  and  larger,  and  forms  a  sac  of  considerable  size, 
filled  with  liquid,  and  in  the  wall  of  which  large  blood- 
vessels form.  It  soon  reaches  the  inner  wall  of  the  egg- 


THE   ALLANTOIS. 


381 


cavity,  and  spreads  itself  out  on  the  inner  surface  of  the 
chorion.  In  many  Mammals  the  allantois  becomes  so  largo 
that  it  finally  surrounds  the  whole  embryo  with  its  other 
appendages,  as  a  great  covering,  and  extends  over  the  whole 
inner  surface  of  the  egg-membrane.  On  cutting  such  an 
egg,  the  first  thing  met  with  is  a  large  space  rilled  with 


FIG.  136. — Embryo  of  Dog,  twenty -five  days  old,  opened  on  the  ventral  side 
(as  in  Figs.  134  and  135).  Chest  and  ventral  walls  have  been  removed  :  a, 
nose-pits;  Z>,  eyes  ;  c,  under  -jaw  (first  gill-arch)  ;  d,  second  gill-arch ;  efgh, 
heart  (e,  right,  /,  left  auricle  ;  g,  right,  h,  left  ventricle) ;  i,  aorta  (origin  of ) ; 
kk,  liver  (in  the  middle  between  the  two  lobes  is  the  cut  yelk-vein);  I, 
stomach ;  TO,  intestine ;  n,  yelk-sac  ;  o,  primitive  kidneys  :  p,  allantois  ;  q, 
fore-limbs  ;  ft,  hind-limbs.  The  crooked  embryo  has  been  stretched  straight. 


382  THE    EVOLUTION    OF   MAN. 

fluid ;  this  is  the  allantois  cavity,  and  it  is  only  after  the 
removal  of  this  membrane  that  the  real  embryonic  body, 
which  is  enclosed  in  the  amnion,  is  found. 

In  Man,  the  allantois  does  not  attain  so  great  a  size, 
but  losing  its  vesicular  form,  changes  into  the  placenta 
soon  after  it  has  reached  the  inner  wall  of  the  chorion. 


FIG.  137.— Embryo  of  a  Dog,  from  the  right  side :  a,  the  first  brain, 
bladder ;  b.  second ;  c,  third ;  d,  fourth ;  e,  the  eye  ;  /,  the  ear-vesicle  ;  gh, 
Srst  gill-arch  (g,  lower  jaw,  h,  upper  jaw)  ;  i,  second  gill-arch ;  Urn,  heart 
(A-,  right  auricle  ;  I,  right  ventricle  ;  m,  left  ventricle)  ;  n,  beginning  of  the 
aorta  ;  o,  heart  pouch  ;  p,  liver  ;  q,  intestine  ;  r,  yelk-duct ;  s,  yelk-sac  (torn 
away)  ;  i,  allantois  (torn  away)  :  u,  amnion  ;  v,  fore-limb ;  x,  hind-limb. 
(After  Bischoff.) 

Yet  even  in  Man  the  first  rudiment  of  the  allantois  is  a 
stalked  pear-shaped  bladder  (Fig.  133,  Z),just  as  in  other 
Mammals.  I  stated  this  in  1874s  in  the  first  and  second 


THE   PLACENTA.  383 

editions  of  this  book,  and  explained  it  in  the  drawing  now 
given  in  Fig.  137.  I  based  the  statement  on  a  very  apt 
deduction.  For  as  the  general  form  and  the  finer  structure 
of  the  placenta  is  entirely  similar  in  Man  and  in  Apes, 
the  origin  of  the  organ  could  not  be  different  in  the  two 
cases.  As,  however,  the  bladder-like  form  of  the  allantois 
of  the  human  being  had  never  been  directly  observed,  I  was 
gravely  accused  by  Wilhelm  His  of  falsifying  science.  His 
stated  that  "  it  is  known  that  the  allantois  in  the  human 
being  is  never  seen  in  the  bladder-like  form "  (!).  Luckily 
for  me,  this  "  never  visible  "  bladder  form  was  actually  seen 
by  Professor  Krause  of  Gb'ttingen  in  the  following  year 
(1875),  and  a  drawing  of  it,  reproduced  in  Fig.  133,  was 
given."" 

When  the  bladder-shaped  human  allantois  has  reached 
the  inner  wall  of  the  tufted  membrane  (choriori),  spreading 
itself  flatly  over  the  latter,  it  forms  the  placenta,  which  is 
very  important  to  the  nourishment  of  the  germ.  The  stalk 
of  the  allantois,  which  connects  the  embryo  with  the 
placenta,  and  carries  the  large  blood-vessels  of  the  navel 
from  the  former  to  the  latter,  is  enveloped  by  the  amnion, 
and,  together  with  the  amnion-sheath,  forms  the  so-called 
navel-cord  (Fig.  138,  a  s).  The  large  network  of  blood- 
tilled  vessels  of  the  embryonic  allantois  attaches  itself 
closely  to  the  mucous  membrane  of  the  maternal  uterus, 
and  the  partition  wall  between  the  blood-vessels  of  the 
mother  and  those  of  the  child  grows  very  much  thinner, 
thus  giving  rise  to  the  remarkable  apparatus  for  nourishing 
the  embryonic  bod}  which  we  call  the  placenta,  and  to 
which  we  shall  refer  hereafter.  (Of.  Chapter  XIX.)  At 
present,  I  will  speak  of  it  only  in  connection  with  the  fact 


3*4 


THE    EVOLUTION    OF   MAN. 


that  it  appears  exclusively  in  the  higher  Mammals,  not  in 
the  lower.  Of  the  three  sub-classes  or  principal  groups  of 
the  Mammals,  the  two  lower  groups,  the  Beaked  Animals 

FIG.  138. — Egg-membranes 
of  the  human  embryo  (diagram- 
matic) :  TO,  the  thick,  fleshy  wall 
of  the  uterus ;  plu,  placenta  (of 
which  the  inner  layer  (plu') 
Bends  processes  in  between  the 
tufts  of  the  chorion  (chz)  ;  chf, 
tufted  chorion ;  chl,  smooth  cho- 
rion ;  a,  amnion ;  ah,  amnion- 
cavity;  as,  amnion-sheath  of 
the  navel-cord  (which  passes 
below  into  the  navel  of  the  em- 
bryo, not  represented  here)  ; 
dg,  yelk-duct ;  ds,  yelk-sac ; 
dv,  dr,  decidua  (dv,  true,  dr, 
false  decidua).  The  cavity  of 
the  uterus  (uli)  opens  below 

into  the  sheath  (vagina),  above,  on  the  right,  into  the  oviduct  (t).     (After 

Kolliker.) 

(Ornithostoma)  and  Pouched  Animals  (Marsupialia),  have  no 
placenta,  the  allantois  remaining  a  simple  bladder,  filled 
with  fluid,  as  in  Birds  and  Reptiles.  Only  in  the  third  and 
most  highly  developed  mammalian  sub-class,  the  Placental 
Animals,  is  a  true  placenta  developed  from  the  allantois. 
To  the  placental  sub-class  belong  the  Hoofed  Animals, 
Whales,  Beasts  of  Prey,  Insect-eating  Animals,  Rodents, 
Bats,  Apes,  and  Men.  This  circumstance  is  direct  evidence 
that  man  has  developed  from  this  group  of  Mammals. 

In  connection  with  the  line  of  descent  of  the  human 
race,  the  allantois  is,  therefore,  of  twofold  interest :  firstly, 
because  this  appendage  is  entirely  wanting  in  the  lower 
classes  of  Vertebrates,  and  is  developed  only  in  the  three 


SIGNIFICANCE   OF   THE   PLACENTA 


385 


FIG.  139. — Five  diagrammatic  longitudinal  sections  through  the  develop- 
ing mammalian  germ  with  its  egg-coverings.  In  Fig.  1-4  the  longitudinal 
section  is  through  the  sagittal  plane  or  the  middle  plane  of  the  body,  which 


386  THE   EVOLUTION   OF  MAN. 

separates  the  right  aud  left  halves  ;  in  Fig.  5  the  germ  is  seen  from  the  left 
side.  In  Fig.  1,  the  prochorion  (d),  studded  with  tufLs  (d'),  surrounds  the 
germ-vesicle,  the  wall  of  which  is  composed  of  the  two  primary  germ- 
layers.  Between  the  outer  (a)  and  the  inner  (t)  germ-layer  within  the' 
limits  of  the  germ-area  (area  germinativa)  the  middle  germ-layer  (mesoderma, 
m)  has  developed.  In  Fig.  2,  the  embryo  (e)  is  already  beginning  to  separate 
from  the  germ-vesicle  (ds),  and  the  wall  of  amnion-fold  is  beginning  to  rise 
round  the  embryo  (in  front  as  the  head-sheath,  ks,  behind  as  the  tail-sheath, 
««.)  In  Fig.  3,  the  edges  of  the  amnion-fold  (am)  meet  over  the  back  of  the 
embryo,  thus  forming  the  amnion-cavity  (ah)  ;  in  consequence  of  the  further 
separation  of  the  embryo  (e)  from  the  germ-vesicle  (els),  the  intestinal- 
canal  (dd)  originates,  and  from  the  hind  end  of  this  the  allantois  (al)  grows 
oat.  In  Fig.  4,  the  allantois  (al)  is  bigger ;  the  yelk-sac  (els)  is  smaller. 
In  Fig.  5,  the  embryo  already  shows  the  gill-openings  and  the  rudiments  of 
the  two  pairs  of  limbs ;  the  chorion  has  formed  branched  tufts.  In  all  five 
figures,  e  indicates  embryo;  a,  outer  germ-layer;  m,  middle  germ-layer; 
i,  inner  germ-layer ;  am,  amnion ;  (ks,  head-sheath ;  as,  tail-sheath) ;  ah, 
amnion-cavity ;  as,  amnion-sheath  of  the  navel-cord ;  kh,  intestinal  germ- 
vesicle ;  ds,  yelk-sac ;  dg,  yelk-duct;  df,  intestinal-fibrous  layer;  eld,  in- 
testinal-glandular  layer ;  al,  allantois ;  vl=hh,  region  of  the  heart ;  d,  yelk- 
membrane  or  prochorion ;  d',  tufts  of  the  latter ;  sh,  serous  covering ;  sz, 
tufts  of  the  latter;  ch,  tufted  membrane  or  chorion;  cht,  tufts  of  the 
latter;  st,  terminal  vein;  r,  cavity,  filled  with  liquid,  between  the  amnion 
and  chorion.  (After  Kolliker.)  (Cf.  PL  V.  Fig.  14  and  15.) 
• 

higher  classes,  in  Reptiles,  Birds,  and  Mammals;  and,  secondly, 
because  the  placenta  is  developed  from  the  allantois  only  in 
the  higher  Mammals,  including  Man,  and  not  in  the  lower 
Mammals.  The  former  are  therefore  called  "Placental 
Animals  "  (Placentalia). 

Another  characteristic  common  to  the  three  higher  classes 
of  Vertebrates  alone,  is  the  formation  of  the  third  appendage 
of  the  embyro,  the  amnion,  which  has  already  been  men- 
tioned. We  have  already  learned  something  of  the  amnion 
ill  noticing  the  separation  of  the  embryo  from  the  intestinal 
germ-vesicle.  We  found  that  the  walls  of  the  latter  rise  in 
a  ring-shaped  fold  round  the  embryonic  body.  In  front,  this 
fold  appears  in  the  form  of  the  so-called  head-cap,  or  head- 


THE  AMNION.  387 

sheath  (Fig.  139,  2  ks) ;  at  the  back,  it  also  arches  upward 
and  forms  the  tail-cap,  or  tail-sheath  (Fig.  139,  2  ss) ;  on  the 
right  and  left  sides,  the  fold  is  at  first  lower,  and  is  here 
called  the  side-caps,  or  side-sheaths  (Fig.  140;  Figs.  95,  96,  af, 
p.  317).  All  these  caps  or  sheaths  are  only  parts  of  a  con- 


FIG.  140. — Transverse  section  through  an  embryonic  Chick  (a  little 
behind  the  anterior  opening  of  the  intestine),  at  the  end  of  the  first  day  of 
incubation.  The  medullary  furrow  above  and  the  intestinal  furrow  below 
are  still  wide  open.  At  each  side,  the  rudiment  of  the  body-cavity  (cceZoma) 
can  be  seen  between  the  skin-fibrous  layer  and  the  intestinal-fibrous  layer. 
On  the  right  and  left  of  it,  at  the  outside,  the  side-caps  of  the  amnion  are 
beginning  to  rise.  (After  Remak.) 

nected  ring-like  fold,  which  passes  round  the  embryo.  This 
grows  higher  and  higher,  rises  like  a  great  encircling  wall,  and 
finally  arches  over  the  body  of  the  embryo,  so  as  to  form  a 
cavern-like  covering  over  the  latter.  The  edges  of  the  ring- 
like  fold  meet  and  coalesce  (Figs.  141,  142):  The  embryo, 
thus,  at  last  lies  in  a  thin  membranous  sac,  filled  with  the 
amnion-fluid  (Fig.  139,  4,  5  ah}. 

When  the  sac  is  completely  closed,  the  inner  layer  of  the 
fold,  which  forms  the  real  wall  of  the  sac,  withdraws  com- 
pletely from  the  outer  layer.  The  latter  attaches  itself  to 
the  inside  of  the  outer  egg-membrane  ("  prochorion ").  It 
supplants  this  prochorion,  and  forms  the  permanent  tufted 
membrane,  the  true  "  chorion."  This  arises  solely  from 
the  horn-plate  (Fig.  139,  4  sk).  The  thin  wall  of  the 
amnion-sac,  on  the  other  hand,  consists  of  two  strata  :  of  an 
inner  stratum,  the  horn-plate,  and  of  an  outer  stratum,  the 


THE   EVOLUTION    OF   MAN. 


skin-fibrous  layer  (Figs.  141, 142).  The  latter  is  indeed  here 
very  thin  and  delicate,  but  yet  can  be  distinctly  shown  to 
be  a  direct  continuation  of  the  leather-skin  (corium),  and 


FIG.  141. — Transverse  section  through  an  embryonic  Chick  in  the  navel 
region  (at  the  fifth  day  of  incubation).  The  amnion-folds  (am)  almost 
meet  over  the  back  of  the  embryo.  The  intestine  (d),  still  open,  passes 
below  into  the  yelk-sac  :  df,  intestinal-fibrous  layer  ;  sh,  notochord  ;  sa, 
aorta ;  vc,  principal  veins ;  Ih,  ventral  cavity,  not  yet  closed ;  v,  anterior, 
g,  posterior  nerve-roots  of  the  spinal  marrow  j  mu,  muscle-plate  ;  hp, 
leather-plate ;  h,  horn-plate.  (After  Kemak.) 

is,  therefore,  the  outermost  layer  arising  from  the  fission  of 
the  middle  germ-layer  (mesoderma).  Thus  the  outer 
peripheric  portion  of  the  skin-fibrous  layer  clothes  only  the 
inner  lamella  of  the  amnion-fold  (the  head-sheath,  tail- 
sheath,  etc.),  and  extends  only  to  the  edge  of  the  fold  itself. 
The  outer  lamella  is  formed  entirely  by  the  horn-plate,  and 


SIGNIFICANCE    OF   THE   AMN1ON. 


389 


it  produces  the  tufted  chorion,  the  hollow,  branched  tufts  of 
which  grow  into  the  depressions  in  the  mucous  membrane 
of  the  maternal  uterus. 


FIG.  142.— Transverse 
section  through  au  em- 
bryonic Chick  in  the 
shoulder  region  (at  the 
fifth  day  of  incubation). 
The  sectionpasses  midway 
between  the  rudiments  of 
the  anterior  limbs  (or 
wings,  E).  The  amnion- 
folds  have  grown  com- 
pletely together  over  the 
back  of  the  embryo. 
(After  Eemak.)  (Com- 
pare,  as  regards  other 
points,  with  Figs.  139, 140, 
and  141,  and  Plate  V. 
Fig.  14.) 


In  human  Phylogeny  the  amnion  is  particularly  in- 
teresting, because  it  is  a  peculiar  characteristic  of  the  three 
higher  classes  of  Vertebrates.  Mammals,  Birds,  and  Reptiles 
alone  possess  it,  and  therefore  these  three  classes  are 
grouped  together  under  the  name  of  Amnion  Animals,  or 
Amniota.  All  Amnion  Animals,  including  Man,  are  de- 
scended from  a  common  parent-form.  All  the  lower  Verte- 
brates, on  the  contrary,  entirely  want  this  amniotic  formation. 

Of  the  three  bladder-like  appendages  of  the  embryo  just 
mentioned,  the  amnion  has  no  blood-vessels  at  any  period 
of  its  existence.  On  the  contrary,  the  two  other  bladders, 
the  yelk-sac  and  the  allantois,  are  provided  with  large  blood- 
vessels, which  accomplish  the  nutrition  of  the  embryonic 


3QO  THE    EVOLUTION   OF   MAN. 

body.  Here  we  may  speak  of  the  first  circulation  of  blood 
in  the  embryo,  and  of  its  central  organ,  the  heart.  The  first 
blood-vessels  and  the  heart,  as  well  as  the  first  blood  itself, 
develop  from  the  intestinal-fibrous  layer.  On  this  account 
the  latter  was  called  the  vascular  layer  by  the  earlier  em- 
bryologists.  In  a  certain  sense  this  name  is  quite  correct ; 
:>nly  it  must  not  be  understood  to  imply  that  all  the  blood- 
vessels of  the  body  proceed  from  this  layer,  or  that  the 
.  whole  of  the  vascular  layer  is  applied  only  to  the  formation 
of  the  blood-vessels.  Neither  is  the  case.  The  intestinal- 
fibrous  layer  also  forms,  as  we  saw,  the  whole  fibrous  and 
muscular  wall  of  the  intestinal  tube,  and  also  the  mesentery. 
We  shall  presently  find  that  blood-vessels  can  form  in- 
dependently from  other  parts,  especially  in  the  various  parts 
proceeding  from  the  skin-fibrous  layer. 

The  heart  and  blood-vessels,  and  the  whole  vascular 
system,  are  by  no  means  among  the  oldest  parts  of  the 
animal  organism.  Aristotle  assumed  that  the  heart  of 
the  Chick  was  formed  first ;  and  many  later  authors  have 
shared  this  view.  This  is,  however,  by  no  means  the  case. 
On  the  contrary,  the  most  important  parts  of  the  body,  the 
four  secondary  germ-layers,  the  intestinal  tube,  and  the 
notochord,  are  already  formed  before  the  first  indication  of 
the  blood-vessel  system  appears.  This  fact,  as  we  shall  after- 
wards find,  is  in  complete  harmony  with  the  Phylogeny  of 
the  animal  kingdom,  Our  older  animal  ancestors  possessed 
neither  blood  nor  heart. 

We  have  already  examined  the  first  blood-vessels  of  the 
mammalian  embryo  in  transverse  sections.  They  are,  first, 
the  two  primary  arteries,  or  "primitive  aortae,"  "which  lie 
hi  the  narrow  longitudinal  cleft  between  the  primitive 


THE   VASCULAR   SYSTEM.  39! 

spinal  cords,  the  side-plates,  and  the  intestinal-glandular 
layer  (Figs.  92,  ao,  Fig.  95,  96,  ao) ;  and,  second,  the  two 
principal  veins,  or  "  cardinal  veins,"  which  appear  somewhat 
later,  outside  the  former,  above  the  primitive  kidney  ducta 
(Fig.  96,  vc,  Fig.  141,  vc).  The  primitive  arteries  seem  to 
arise  by  fission  from  the  inner  parts  of  the  intestinal-fibrous 
layer ;  the  primitive  veins,  on  the  contrary,  from  the  outer 
parts  of  the  same  layer. 

In  just  the  same  way,  and  in  connection  with  these  first 
blood-vessels,  the  heart  also  arises  from  the  mtestinal- 
fibrous  layer,  in  the  lower  wall  of  the  anterior  intestine, 
near  the  throat,  at  the  place  where  the  heart  remains 
throughout  life  in  Fishes.  Perhaps  it  will  not  seem  very 
poetic  that  the  heart  develops  directly  from  the  intestinal 
wall  But  the  fact  cannot  be  altered,  and  is  also  easily 
comprehensible  phylogenetically.  The  Vertebrates  are,  at' 
any  rate,  in  this  respect  more  aesthetic  than  the  Mussels. 
In  these  the  heart  remains  permanently  lying  behind  on  the 
waU  of  the  rectum  near  the  anus,  so  that  the  heart  seems  to 
be  penetrated  by  the  rectum. 

Midway  between  the  gill-arches  of  the  two  sides  of  the 
head,  and  rather  further  back,  at  the  throat  of  the  embryo,  a 
wart-like  thickening  of  the  intestinal-fibrous  layer  develops 
on  the  lower  wall  of  the  intestinal  head  cavity  (Fig.  143,  df}. 
This  is  the  first  rudiment  of  the  heart.  This  swelling  is 
spindle-shaped,  at  first  quite  solid,  and  is  formed  entirely  of 
cells  of  the  intestinal-fibrous  layer.  It  afterwards,  however, 
curves  in  the  form  of  an  S  (Fig.  144,  c),  and  a  little  hollow 
is  formed  in  its  centre,  in  consequence  of  the  accumulation 
of  a  small  quantity  of  fluid  between  the  central  cells. 
Some  single  cells  of  the  wall  separate  from  the  rest  and 


392  THE   EVOLUTION   OF   MAN. 

float  about  in  this  fluid.     These  cells  are  the  first  blood- 
cells,  and  the  fluid  is  the  first  blood.     In  the  same  way 


l-k 


FIG.  143. — Longitudinal  section  through  the  head  of  an  embryonic  Chick 
(at  the  end  of  the  first  day  of  incubation)  :  711,  medullary  tube;  ch,  noto- 
chord ;  d,  intestinal  tube  (with  a  blind  anterior  end) ;  fc,  head-plates  ;  df, 
first  rudiment  of  the  heart  (in  the  intestinal-fibrous  layer  of  the  ventral 
wall  of  the  head  intestine) ;  hh,  cavity  for  the  heart ;  hit,  membrane  of  the 
heart ;  kTc,  head-cap  of  the  amnion ;  ks,  head-sheath ;  h,  horn-plate.  (After 
Eemak.) 

FIG.  144. — Human  embryo,  of  14  to  18  days,  opened  at  the  ventral  side. 
Under  the  forehead-process  of  the  head  (t)  the  heart  (c)  appears  in  the 
heart-cavity  (p)  with  the  base  of  the  aorta  (b).  The  greater  part  of  the 
yelk-sac  (o)  has  been  removed  (at  x,  the  opening  of  the  anterior  intestine) ; 
g,  the  primitive  aortee  (lying  under  the  primitive  vertebrae)  ;  ?',  terminal 
intestine,  or  large  intestine  ;  a,  allantois  («,  its  stalk) ;  v,  amnion.  (After 
Coste.) 


THE   BLOOD 


393 


blood    arises   in   the   first   rudimentary   blood-vessels   con- 
nected with  the  heart.     These  also,  at  first,  are  solid,  round 


FIG.  145. — Transverse  section  through  the  head  of  an  embryonic  Chick 
of  36  hours.  Below  the  medullary  tube,  the  two  primitive  aortse  (pa)  are 
visible  in  the  head-plates  (s)  on  both  sides  of  the  notochord.  Below  the 
throat  (d)  can  be  seen  the  aortal-end  of  the  heart  (ae) ;  hh,  heart-cavity  ; 
hk,  heart  membrane ;  ks,  head-sheath,  amnion-fold ;  h,  horn-plate.  (After 
Remak.) 

FIG.  146. — -Transverse  section  through  the  heart-region  of  the  same 
Chick  (further  back  than  the  former).  In  the  heart-cavity  (hK),  the  heart 
(/i)  is  still  connected  by  a  heart-mesentery  (%)  with  the  intestinal-fibrons- 
layer  (df)  of  the  anterior  intestine  :  d,  intestinal-glandular  layer ;  up, 
primitive  vertebral  plates;  gb,  rudiment  of  the  ear-vesicle  in  the  horn- 
plate  ;  hp,  first  rising  of  the  amnion-fold.  (After  Remak.) 

cords  of  cells.  They  then  become  hollow,  while  a  fluid 
separates  and  gathers  in  the  centre,  and  single  cells  detach 
themselves  from  the  rest  and  become  blood-cells.  This  is 
equally  true  of  the  arteries,  which  carry  the  blood  from  the 


394  THE   EVOLUTION   OF   MAN. 

heart,  and  of  the  veins,  which  carry  the  blood  back  to  the 
heart. 

At  first,  the  heart  lies  within  the  intestinal  wall  itself, 
from  which  it  has  developed,  as  do  the  first  main  blood- 
vessels proceeding  from  it.  The  heart  itself  is  in  reality 
only  a  local  extension  of  one  of  these  main  blood-vessels. 
Soon,  however,  the  heart  separates  from  its  place  of  origin, 
and  now  lies  freely  in  a  cavity,  called  the  heart-cavity 
(Figs.  145,  hh,  146,  hh).  This  heart-cavity  is  merely  the 
anterior  part  of  the  body-cavity  (cceloma),  which,  as  a 
horseshoe-shaped  arch,  connects  the  right  and  left  divisions 
of  the  ccelom  (Fig.  140).  The  wall  of  the  heart-cavity  is 
therefore  formed,  like  that  of  the  remainder  of  the  body- 
cavity,  partly  by  the  intestinal-fibrous  layer  (Fig.  146,  df], 
and  partly  by  the  skin-fibrous  layer  Qip).  While  the  heart 
is  separating  from  the  anterior  intestine,  it  remains  for  a 
short  time  attached  to  the  latter  by  a  thin  plate,  a  heart- 
mesentery  (Fig.  146,  Jig).  It  afterwards  lies  quite  freely  in 
the  heart-cavity,  and  is  directly  connected  with  the  intestinal 
wall  only  by  the  main  blood-vessels  which  pass  from  it. 

The  anterior  extremity  of  this  spindle-shaped  heart- 
formation,  which  soon  assumes  a  curved,  S-shaped  form, 
divides  into  a  right  and  a  left  branch.  These  two  tubes 
are  arched  and  curved  upward,  and  represent  the  two  first 
aortse-arches.  They  mount  up  in  the  wall  of  the  anterior 
intestine,  which,  in  a  measure,  they  encircle,  and  they  there 
unite  above  at  the  upper  wall  of  the  intestinal  head-cavity 
in  one  large  single  main  artery,  which  passes  backward 
immediately  under  the  notochord,  and  which  is  called  the 
main  aorta  (aorta  principalis,  Fig.  147,  a).  The  first  pair 
of  aortse-arches  passes  up  on  the  inner  wall  of  the  first  pail 


DEVELOPMENT  OF  THE  HEART. 


395 


of  gill-arches,  and  lies,  therefore,  between  the  first  gill-arch 
(&)  on  the  outside,  and  the  anterior  intestine   (cZ)  on  the 


Cll 


FIG.  147. — Diagrammatic 
transverse  section  through 
the  head  of  an  embryonic 
Mammal  -.  Ji,  horn-plate ;  m, 
medullary  tube  (brain-blad- 
der) ;  mr,  wall  of  the  latter ; 
7,  leather-plate;  s,  rudiment- 
ary skull;  ch,  notochord; 
fc,  gill-arch ;  mp,  muscle- 
plate  ;  c,  heart-cavity,  an- 
terior part  of  the  body- 
cavity  (cceloma) ;  d,  in- 
testinal tube;  dd,  intes- 
tinal-glandular layer ;  df, 
intestinal -muscle  plate;  Tig, 
heart-mesentery ;  hw,  heart- 
wall  ;  hk,  ventricle ;  ab, 
aorta-arches ;  a,  transverse 
section  throuf't  the  aorta. 


inside,— just  as  these  vascular  arches  are  situated  in  adult 
fishes  throughout  life.  The  single  main  aorta,  which  results 
from  the  union  above  of  these  two  first  vascular  arches, 
soon  again  divides  into  two  parallel  branches,  which  pass 
backward  on  both  sides  of  the  notochord.  These  are  the 
primitive  aortse,  which  have  been  already  spoken  of;  they 
are  also  called  posterior  vertebral  arteries  (arterice  verte- 
brates posteriores).  These  two  main  arteries  send  out  on 
each  side  from  four  to  five  branches  at  right  angles,  which 
pass  from  the  body  of  the  embryo  into  the  germ-area,  and 
are  called  the  omphalic-mesenteric  arteries  (arterice  omphalo- 
mesentericce},  or  the  yelk-arteries  (arterice  vitellince). 
They  represent  the  first  rudiments  of  a  circulation  within 
the  germ-area.  The  first  blood-vessels,  therefore,  pass  out 
from  the  body  of  the  embryo  and  extend  to  the  edge  of 


396 


THE    EVOLUTION    OF   MAN. 


the  germ-area.  Numerous  blood-vessels  form  in  the  intes- 
tinal-fibrous layer  of  the  germ-area.  They  are  at  first 
confined  to  the  dark  germ-area,  or  the  so-called  "  vascular 


Fro.  148.— Canoe-shaded 
germ  of  a  Dog,  from  the 
ventral  side  ;  enlarged 
about  10  times.  In  front, 
below  the  forehead,  the 
first  pair  of  gill-arches  are 
visible;  below  these  is  the 
S-shaped  bent  heart,  close 
by,  and  on  either  side  of 
which  lie  the  two  ear-vesi- 
cles. Posteriorly,  the  heart 
divides  into  the  two  yelk- 
veins,  which  spread  them- 
selves over  the  germ-area 
(the  greater  part  of  this  has 
been  torn  away).  At  the 
bottom  of  the  open  ventral 
cavity  the  primitive  aorta? 
lie  between  the  primitive 
vertebrae,  and  from  which 
five  pairs  of  yelk-artcrics 
proceed.  (After  Bischoff.) 


area  "  (area  opaca,  or  area  vasculosa) ;  but  they  afterwards 
extend  over  the  whole  outer  surface  of  the  intestinal  germ- 
vesicle.  The  whole  yelk-sac,  finally,  seems  to  be  enveloped 
in  a  network  of  blood-vessels.  It  is  the  function  of  these 
blood-vessels  to  collect  food-material  from  the  contents  of 
the  yelk-sac  and  carry  it  to  the 'body  of  the  embryo.  This 
is  done  by  veins,  by  blood-vessels  leading  back,  which  pass 
in  at  the  posterior  opening  of  the  heart,  first  from  the  germ- 
area  and  later  from  the  yelk-sac.  These  veins  are  called 
yelk-veins  (vence  vitellincK);  they  are  also  often  called 
omphalic-mesenteric  vein's  (vence  omphalo-mesentericce'). 


PRIMITIVE  CIRCULATORY   SYSTEM. 


397 


Thus  the  first  circulatory  system  of  the  blood  in  the 
embryo  (Figs.  148-150)  occurs  in  all  the  higher  classes  of 


FIG.  149. — Embryo  and  germ-area  of  a  Rabbit,  in  which  the  earliest 
rudiments  of  the  blood-vessels  appear,-  -seen  from  the  ventral  side  (magni- 
fied about  ten  times).  The  posterior  end  of  the  simple  heart  (a)  divides 
into  two  large  yelk-veins,  which  form  a  network  of  blood-vessels  on  the 
dark  germ-area  (which  appears  light  on  the  black  background).  At  the 
head  extremity  the  fore-brain  with  the  two  eye-vesicles  (bb)  may  be  seen. 
The  dark  centre  of  the  germ  is  the  wide-open  intestinal  cavity.  Ten 
primitive  vertebras  are  visible  on  each  side  of  the  notochord.  (After 
Bischoff.) 

Vertebrates  in  the  following  simple  order.  The  very  simple 
pouch-shaped  heart  (Fig.  150,  d)  divides  both  in  front  and 
behind  into  two  vessels.  Those  at  the  back  are  veins 
leading  to  the  heart.  They  take  food-material  from  the 
germ- vesicle,  or  yelk- sac,  and  carry  it  to  the  body  of  the 


398  THE   EVOLUTION   OF   MAN. 

embryo.  The  vessels  passing  from  the  heart  in  front  are  the 
gill-arch  arteries,  leading  from  the  heart,  and  which,  rising 
as  aorta-arches,  encircle  the  anterior  end  of  the  intestine, 
and  unite  in  the  main  aorta  (aorta  principalis).  The  two 
branches,  which  result  from  the  division  of  this  main  artery, 


FIG.  150. — Embryo  and  germ-area  of  a  Rabbit,  in  which  the  first  system 
of  blood-vessels  is  complete, — seen  from  the  ventral  side  (magnified  aboujb 
five  times).  The  posterior  end  of  the  heart  (d),  which  is  curved  in  the  form 
of  an  S,  divides  into  two  large  yelk-veins,  each  of  which  sends  out  an 
anterior  branch  (b)  and  a  posterior  branch  (c).  The  ends  of  these  unite  in 
the  circular  boundary  vein,  or  terminal  vein  (v.  terminalis)  (a).  In  the  germ- 
area  may  be  seen  the  coarser  venous  network  (lying  below),  and  the  finer 
arterial  network  (lying  nearer  the  surface).  The  yelk-arteries  (/)  open 
into  the  two  primitive  aortae  (e).  The  dark  area  which  surrounds  the  head 
like  a  halo,  represents  the  recess  within  the  head-cap  or  membrane. 
(After  Bischoff.) 


SECONDARY   CIRCULATORY    SYSTEM.  399 

the  primitive  aortae,  send  out  right  and  left  the  yelk-arteries, 
which  leave  the  body  of  the  embryo  and  pass  into  the  germ- 
area.  Here,  and  in  the  circumference  of  the  navel-vesicle, 
two  layers  of  vessels  are  distinguishable — the  superficial 
arterial  layer,  and  the  lower  venous  layer.  The  two  are 
connected  together.  At  first  this  system  of  blood-vessels 
is  extended  only  over  the  superficial  front  of  the  germ-area 
as  far  as  the  edge.  Here,  on  the  edge  of  the  dark  vascular 
area,  all  the  branches  unite  in  a  large  terminal  vein  (vena 
terminalis,  Fig.  150,  a).  This  vein  disappears  at  a  later 
period,  as  soon  as;  in  the  course  of  development,  the  for- 
mation of  blood-vessels  progresses  further,  and  then  the 
yelk-vessels  traverse  the  whole  yelk-sac.  When  the  navel- 
vesicle  degenerates,  these  vessels,  of  course,  also  degenerate, 
being  of  importance  only  in  the  first  period  of  embryonic 
life. 

This  first  circulation  in  the  yelk-sac  is  replaced,  at  a 
later  period,  by  the  second  circulation  of  blood  in  the 
embryo,  that  of  the  allantois.  Large  blood-vessels  are 
developed  on  the  wall  of  the  primitive  urinary  sac,  or 
allantois,  from  the  intestinal-fibrous  layer.  These  vessels 
grow  larger  and  larger,  and  are  most  intimately  connected 
with  the  vessels  that  develop  in  the  body  of  the  embryo 
itself.  This  secondary  allantois  circulation  thus  gradually 
takes  the  place  of  the  original,  primary,  yelk-sac  circulation. 
When  the  allantois  has  grown  to  the  inner  wall  of  the 
chorion,  and  has  changed  itself  into  the  placenta,  its  blood- 
vessels alone  accomplish  the  nourishment  of  the  embryo. 
They  are  called  navel- vessels  (vasa  umbilicalia),  and  are 
originally  in  pairs :  one  pair  of  navel  arteries,  and  one  pair 
of  navel  veins.  The  two  navel- veins  (venae  umbilicale^ 


4OO  THE   EVOLUTION   OF   MAN. 

Figs.  123,  U,  124,  u),  which  carry  blood  from  the  placenta  to 
the  heart,  open,  at  first,  into  the  united  yelk-veins.  These 
last  afterwards  disappear,  and  the  right  navel-vein  siimul- 
taneously  disappears  entirely,  so  that  a  single  great  vein, 
the  left  navel-vein,  alone  remains,  which  carries  all  the 
nutritive  blood  from  the  placenta  into  the  heart  of  t!ie 
embryo.  The  two  arteries  of  the  allantois,  or  the  navel  - 
arteries  (arterice  umbilicales,  Figs.  123,  n,  124,-n,),  are  merely 
the  last,  posterior  extremities  of  the  two  primitive  aortas, 
which  are  afterwards  greatly  developed.  It  is  not  until  the 
end  of  the  nine  months  of  embryonic  life,  when  the  human 
embryo  is  born  and  enters  the  world  as  an  independent 
physiological  individual,  that  the  navel  circulation  loses  its 
significance.  The  navel  cord  (Fig.  138,  as),  in  which  these 
larger  blood-vessels  pass  from  the  embryo  to  the  placenta, 
is  removed  with  the  latter  at  the  so-called  "after-birth," 
and  an  entirely  new  circulation  of  the  blood,  limited  to  the 
body  of  the  child,  comes  into  operation  simultaneously  with 
pulmonary  respiration.107 

Now,  if,  in  conclusion,  we  briefly  review  the  germ- 
history  of  Man  as  far  as  we  have  traced  it,  and  endeavour 
to  comprehend  the  whole  subject  in  one  connected  view,  it 
seems  desirable  to  divide  it  into  several  main  sections,  or 
periods,  and  these  into  subordinate  stages,  or  »t<;ps.  With 
reference  to  the  phylogenetic  significance  of  this  hibtory. 
which  we  shall  next  consider  more  closely,  it  seems  to  me 
most  appropriate  to  make  the  four  main  divisions  and  ten 
sub-divisions  as  distinguished  in  the  following  pages,  which 
correspond  to  the  most  important  phylogenetic  stages  of 
development  of  our  animal  ancestors.  (Cf.  Table  XXV. 
at  the  end  of  the  nineteenth  chapter.)  This  will  again 


TERTIARY   CIRCULATORY    SYSTEM.  4OI 

shew  that  the  germ-history  of  Man  (according  to  the  law 
of  abbreviated  heredity)  is  very  rapid  and  compressed  in 
the  first  stages  of  its  course,  but  grows  slower  and  slower 
in  each  succeeding  stage.  All  the  remarkable  phenomena 
which  we  observe  in  the  transformation  of  the  human 
embryo  in  the  whole  course  of  our  Ontogeny,  are  intel- 
ligible only  with  the  help  of  Phylogeny,  and  are  explicable 
only  by  reference  to  the  historical  metamorphoses  of  our 
animal  ancestry.108 

It  is  true  that  if  the  ontogenetic,  and  the  phylogenetic 
stages  (in  Tables  VIII.  and  XXII.)  are  carefully  compared, 
a  complete  agreement  between  the  two  is  not  observable ; 
on  the  contrary,  there  are  many  individual  divergences.  In 
germ-history  many  organs  appear  earlier,  others  later, 
than  the  probable  course  of  tribal  history  leads  us  to 
expect.  But  an  adequate  explanation  of  these  divergences 
is  found  in  the  various  kenogenetic  modifications  which 
the  germ-history  of  the  higher  Vertebrates  has  undergone 
in  the  long  course  of  its  evolution.  This  will  become  quite 
clear  when  we  carefully  compare  the  germ-history  of  Man 
with  the  Ontogeny  of  the  lowest  Vertebrate,  the  Amphioxus. 
an  Ontogeny  distinguished  by  tenacious  inheritance  of  the 
original  course  of  evolution. 


TABLE     VIII. 

SYSTEMATIC  SURVEY  OF  THE  PERIODS  IN  HUMAN  GERM-HISTORY. 
(Cf.  Table  XXII.) 


FIRST  MAIN  DIVISION  OF  GERM-HISTORY. 

Kan  as  a  simple  Flastid. 

The  human  embryo  possesses  the  form-value  of  u  simple  individual  of  the 
firot  order  of  a  single  plastid. 

First  Stage  :  Monerula  Stage  (Fig.  36,  p.  210). 

The  human  germ  is  a  simple  cytod  (the  impregnated  egg-cell  after  the 
loss  of  the  germ- vesicle). 

Second  Stage:  Cytula  Stage  (Fig.  37, p.  210). 

The  human  germ  is  a  simple  cell  (the  impregnated  ovulc-ocll  with  the 
re-formed  kernel,  or  the  parent-cell). 

SECOND  MAIN  DIVISION  OF  GERM-HISTORY. 

VIOL  as  a  many-celled  Primitive  Amtpnl. 

The  human  embryo  consists  of  many  cells,  which,  however,  nr  jet  form 
110  organs;  it  therefore  possesses  the  form-value  of  an  individual  of  the 
second  order. 

Third  Stage:  Morula  Stage  (Fig.  40,  p.  212,  and  PI.  II.  Fig.  14). 
The  human  germ  is  a  globular  cell-mass,  of  which  one  hemisphere  consists 
of  animal  cells,  the  other  of  vegetative  cells. 

Fourth  Stage  :  Blastula  Stage  (PI.  II.  Fig.  16). 

The  human  germ  is  a  vesicle,  the  wall  of  which  consista  of  animal  cells, 
its  contents  of  vegetative  c»«*. 


HUMAN   GERM   HISTORY.  403 

THIRD  MAIN  DIVISION  OP  GERM-HISTORY. 

Han  as  an  invertebrate  Intestinal  Animal. 

The  human  embryo  possesses  the  form- value  of  an  individual  of  the  third 
order,  an  unarticulated  person  (a  single  metameron).  The  primitive  in- 
testinal cavity  is  enclosed  by  two  primary  germ-layers,  from  the  fission  of 
which  four  secondary  germ-layers  are  presently  formed. 

Fifth  Stage  :  Gastrula  Stage  (Fig.  41,  p.  213,  and  PL  II.  Fig.  17). 
The  human  germ  forms  an  Amphigastrnla,  consisting  solely  of  the  two 
primary  germ-layers,  the  skin-layer,  and  the  intestinal  layer.     The  cavity  of 
the  primitive  intestine  is  occupied  by  entoderm  cells,  which  also  plug  the 
primitive  mouth. 

Sixth  Stage  :  Chordonium  Stage  (Fig.  90,  p.  302). 

The  human  germ  possesses,  in  all  essential  points,  the  organization  of  a 
worm,  of  which  the  nearest  existing  allied  form  seems  to  be  the  ascidian 
larva.  Four  secondary  germ-layers  have  developed  from  the  two  primary 
germ-layers,  and  coalesce  along  the  central  line. 


FOURTH  MAIN  DIVISION  OF  GERM-HISTORY. 

Man  as  a  true  Vertebrate. 

The  human  embryo  possesses  the  form-value  of  an  articulated  person, 
or  a  metameric  chain.  The  articulation  principally  affects  the  bony 
system  (primitive  vertebras)  and  the  muscle-system.  The  skin-sensory 
layer  is  divided  into  the  horn-plate,  the  medullary  tube,  and  the  primitive 
kidneys.  The  skin-fibrous  layer  has  separated  into  the  leather-plate,  the 
primitive  vertebra  (muscle-plate  and  bone-plate),  and  the  notochord.  From 
the  intestinal-fibrous  layer  proceed  the  heart  with  the  principal  blood- 
vessels, and  the  fleshy  intestinal  wall.  From  the  intestinal-glandular  layer 
the  epithelium  of  the  intestinal  tube  is  formed. 

Seventh  Stage  :  Acranial  Stage  (Figs.  103,  107,  pp.  342,  344). 
The  human  germ  possesses,  in  essential  points,  the  organization  of  a  skull- 
less  vertebrate,  similar  to  the  developed  Amphioxus.  The  body  already 
forms  a  chain  of  metamera,  as  several  primitive  vertebra  have  become 
distinct.  The  head  is,  however,  not  yet  distinctly  separated  from  the 
trunk.  The  medullary  tube  has  not  yet  differentiated  into  the  brain- 
bladders.  The  skull  is  still  wanting,  as  are  also  the  heart  and  limbs. 


404  THE   EVOLUTION    OF    MAN. 

Eighth  Stage  :  Cyclostoma  Stage  (Fig.  132,  p.  377,  PI.  VII.  Fig.  M  L> 
The  human  germ  possesses,  in  essential  points,  the  organization  of  a  gill- 
less  cranial  animal  (like  the  developed  Myxiuoida  and  Petromyzonta).  The 
number  of  metamera  is  increasing.  The  head  is  more  distinctly  differenti- 
ated from  the  trunk.  The  anterior  extremity  of  the  medullary  tube  shells 
in  the  form  of  a  bladder,  and  forms  the  rudimentary  brain,  which  soon 
divides  into  five  brain-bladders,  lying  one  behind  the  other.  On  the  sides  of 
these  appear  the  rudiments  of  the  three  higher  sense-organs :  the  nose-pit, 
and  the  eye  and  ear  vesicles.  With  the  first  circulation  of  the  blood  the 
heart  begins  its  activity.  The  jaws  and  limbs  are  still  wanting. 

Ninth  Stage  :  Ichthyod  Stage  (Fig.  134,  p.  378,  PI.  VII.  Pig.  M  II.). 
The  human  germ  possesses,  in  essential  points,  the  organization  of  a  fish 
(or  a  fish-like  Skulled-animal).  The  two  pairs  of  limbs  appear  in  the  simplest 
form,  as  fin-like  processes :  a  pair  of  anterior  limbs  (dorsal  fins)  and  one 
pair  of  posterior  limbs  (ventral  fins).  The  gill-openings  are  completely 
formed,  and  between  these  the  gill-arches  form;  the  first  pair  of  gill -arches 
differentiate  into  the  rudiments  of  the  upper  and  lower  jaws.  From  the  in. 
testinal  canal  proceed  lungs  (swimming-bladder),  liver,  and  pancreas. 

Tenth  Stage :  Amniotio  Stage  (PI.  VII.  Fig.  M  III. ;  PI.  VIII.). 
The  human  germ  possesses,  in  essential  points,  the  organization  of  an 
Amnion-animal  (of  a  higher  gill-loss  Vertebrate).  The  gill-openings  disap- 
pear by  concrescence.  From  the  gill-arches  develop  the  jaws,  the  tongue- 
bone,  and  the  bonelets  (ossicles)  of  the  ear.  The  allantois  perfects  itself, 
and  changes  into  the  peripherio  portion  of  the  placenta.  All  the  organs 
gradually  acquire  the  forms  peculiar  to  the  mammals,  and  at  last  the 
specific  human  form.  (Compare  on  these  points  the  Phyloguuy  iu  the 
following  chapters.109) 


HAKCKEL'S  EVOLUTION  OF  MAN. 


PLATE  VIII 


(    405     ) 
EXPLANATION  OF  PLATES  VIII.  AND  IX. 

(Both  Plates  are  copied  from  Erdl,  "  Entwickelung  des  Menschen"  1I0) 
PLATE  VIII.  FIG.  1. — A  human  embryo  of  nine  weeks,  taken  out  from  tbo 
egrg-membranes  and  magnified  three  times.  (Erdl,  Plate  XII.  Pig.  1-5.) 
The  skull  is  still  quite  transparent,  so  that  the  different  divisions  of  the 
brain  show  through  :  tbe  large  mid-brain  (''  four-bulbs  ")  is  separated  from 
the  scarcely  larger  fore -brain  (cerebrum)  by  a  shallow  groove,  but  from  the 
smaller  hind-brain  (cerebellum)  by  a  deep  indentation.  The  forehead  is 
much  arched  in  front ;  the  nose  is  yet  very  undeveloped ;  the  eye  is  still  dis- 
proportionately large  and  wide  open.  The  upper  lip  is  still  very  short  and 
thickly  swollen ;  the  under  lip  is  very  thin  ;  the  chin  is  short  and  very  re- 
treating. The  whole  face  is  very  small  in  proportion  to  the  skull.  The  ear- 
shell  is  also  very  small,  but  the  outer  opening  of  the  ear  very  large.  The 
neck  is  still  very  short ;  the  trunk,  only  about  a  third  longer  than  the  head, 
is  of  uniform  thickness,  and,  towards  the  tail,  is  produced  into  a  blunt  point. 
The  two  pairs  of  limbs  are  already  completely  articulated.  The  anterior 
limbs  (arms)  are  somewhat  shorter  than  the  posterior  limbs.  The  upper 
and  lower  parts  of  the  arm  (arm  and  foie-arm)  are  very  short  in  proportion 
to  the  hand,  and,  similarly,  the  tipper  and  lower  parts  of  the  leg  (thigh-bone 
and  leg-bone)  are  short  in  proportion  to  the  foot.  The  fingers  on  the  hand 
are  almost  complete ;  while,  on  the  contrary,  the  toes  on  the  foot  are 
completely  bound,  as  far  as  the  points,  in  a  swimming  membrane,  BO  that 
they  form  fins. 

PLATE  VIII.  FIG.  2. — A  human  embryo  of  twelve  weeks,  within  the  egg- 
membranes  ;  natural  size.  (Erdl,  Plate  XI.  Fig.  2.)  The  embryo  is  com- 
pletely enclosed  in  the  amnion,  which  is  filled  with  the  amnion  fluid,  as  in  a 
water-bath.  The  navel  cord,  which  passes  from  the  navel  of  the  embryo  to 
the  chorion,  is  sheathed  in  a  continuation  of  the  amnion,  which  forms  folds 
at  the  point  where  it  is  fastened.  Above,  the  closely-crowded  and  branched 
chorion-tnfts  form  the  placenta.  The  lower  part  of  the  chorion  (cut  open 
and  laid  in  many  small  folds)  is  smooth  and  destitute  of  tufts.  Below  it, 
the  "decidua,"  which  is  also  cut  and  spread  out,  is  still  hanging  in  deepei 
folds.  The  head  and  limbs  of  the  embryo  are  already  considerably  more 
developed  than  in  Fig.  1. 

PLATE  IX. — A  human  embryo  of  five  months ;  natural  size.  (Erdl,  Plate 
XIV.)  The  embryo  is  enclosed  in  the  delicate  transparent  amnion,  whicl- 
has  been  cut  open  in  front,  so  that  the  face  and  limbs  are  plainly  seen 
through  the  opening.  The  back  is  bent,  the  limbs  drawn  together,  so  that 
the  embryo  occupies  the  smallest  possible  space  in  the  egg-cavity.  The 
eyelids  are  closed.  From  the  navel  the  thick  navel-cord  passes,  in  ser- 
pentine folds,  over  the  right  shoulder  to  the  back,  and  from  there  to  the 
spongy  placenta  (below,  on  the  right).  The  outer,  thin,  much -folded  cover- 
Ing  is  the  outer  egg-membrane,  the  chorion.11' 


CHAPTEK  XIII. 

THE   STRUCTURE   OF  THE   BODY  OF   THE  AMPHIOXUS 
AND   OF   THE  ASCIDIAN. 

Cansal  Significance  of  the  Fundamental  Law  of  Biogeny. — Influence  of 
Shortened  and  Vitiated  Heredity. — Kenogenetic  Modification  of  Palin- 
genesis.— The  Method  of  Phylogeny  based  on  the  Method  of  Geology. — 
Hypothetic  Completion  of  the  Connected  Evolutionary  Series  by  Appo- 
sition  of  the  Actual  Fragments. — Phylogenetic  Hypotheses  are  Reliable 
and  Justified. — Importance  of  the  Amphioxus  and  the  Ascidian. — 
Natural  History  and  Anatomy  of  the  Amphioxus. — External  Structure 
of  the  Body. — Skin-covering. — Outer-skin  (Epidermis)  and  Leather-skin 
(Corium). — Notochord. — Medullary  Tube. — Organs  of  Sense. — Intestine 
with  an  Anterior  Respiratory  Portion  (Gill-intestine)  and  a  Posterior 
Digestive  Portion  (Stomach-intestine). — Liver. — Pulsating  Blood-vessels. 
—Dorsal  Vessel  over  the  Intestine  (Gill-vein  and  Aorta)  .—Ventral 
Vessel  under  the  Intestine  (Intestinal  Vein  and  Gill-artery). — Move- 
ment of  the  Blood. — Lymph-vessels. — Ventral  Canals  and  Side  Canal? 
• —  Body-cavity  and  Gill-cavity.  —  Gill-covering.  —  Kidneys.  —  Sexual 
Organs. — Testes  and  Ovaries. — Vertebrate  Nature  of  Amphioxus. — Com- 
parison of  Amphioxus  and  Young  Lamprey  (Petromyzon). — Comparison 
of  Amphioxus  and  Ascidian. — Cellulose  Tunic. —  Gill-sac. — Intestine. 
— Nerve-centres. — Heart. — Sexual  Organs. 

"  The  primitive  history  of  the  species  is  all  the  more  fully  retained  in 
its  germ-history  in  proportion  as  the  series  of  embryonic  forms  traversed  is 
longer ;  and  it  is  more  accurately  retained  the  less  the  mode  of  life  of  the 
recent  forms  differs  from  that  of  the  earlier,  and  the  less  the  peculiarities 
of  the  several  embryonic  states  must  oe  regarded  as  transferred  from  » later 
to  an  earlier  period  of  life,  or  as  acquired  independently." — FKITZ  MULLEK 
(1864). 

IN  turning   from   the   germ-history  to   the  tribal  history 
of  Man,  we  must  constantly  bear  in  mind  the  causal  connec« 


THE   TRIBAL    AND   GERM-HISTORY   OF   MAN.  407 

tion  which  exists  between  these  two  main  branches  of  the 
history  of    human  evolution.   -We  found   that   this   most 
significant  causal  connection  was  most  simply  expressed  in 
"  the  fundamental  law  of  organic  evolution,"  the   meaning 
and  significance  of  which  was  explained  in  detail  in  the 
first  chapter.     According  to  that  first  biogenetic  principle, 
Ontogeny    is    a   short    and    compressed    recapitulation   of 
Phylogeny.      If  this  reproduction  of  tribal  history  were 
always  complete  in  germ-history,  it  would  be  an  easy  task 
to  re-arrange  Phylogeny  by   using  Ontogeny  as   a  guide. 
When  any  one  wanted  to  know  from  what  ancestors  each 
higher   organism  is  descended,   therefore  also   from  what 
ancestors  Man  is  descended,  and  from  what  forms  the  whole 
human  race  has  developed,  it  would  only  be  necessary  to 
trace  accurately  the  series  of  forms   which   occur  in  the 
evolution  of  the  individual  from  the  egg;  each  form  occur- 
ring in  this  series  might  then,  without  further  trouble,  be 
regarded  as   the    representative    of    an    old    and    extinct 
ancestral  form.     But,  as  a  matter  of  fact,  this  immediate 
translation  of  ontogenetic  facts  into  phylogenetic   concep- 
tions is   only   directly  allowable   in  the   case   of  a  com- 
paratively small  part  of  animals.     There  are,  it  is  true,  a 
number  of  low,  Invertebrate  Animals  (e.g.,  Plant-animals, 
Worms,  Crabs)  still  extant,  each  germ-form  of  which  we  are 
justified  in   explaining,   without    further   trouble,  as    the 
reproduction,  or  the  portrait,  of  an  extinct  parent-form.     But 
in  most  animals,  and  in  Man,  this  is  impossible,  because 
the  germ-forms  themselves  have  again  been  modified,  and 
have  partly  lost  their  original  nature,  in  consequence  of  the 
infinite  variety  in  the  conditions  of  existence. 

During  the  immeasurable  course  of  the  organic  history 


4.08  THE    EVOLUTION    OF    MAN. 

of  the  earth,  during  the  many  millions  of  years,  in  the 
course  of  which  organic  life  has  been  developing  on  our 
planet,  modifications  in  the  mode  of  germination  have 
occurred  in  most  animals ;  this  fact  was  first  clearly  recog- 
nized by  Fritz  Miiller-Desterro,  and  was  thus  expressed 
in  his  able  work,  "  Fur  Darwin."  "  The  historical  record 
preserved  in  the  history  of  the  evolution  (of  an  individual) 
is  gradually  obliterated,  in  consequence  of  the  fact  that 
evolution  continually  strikes  out  a  straighter  road  from 
the  egg  to  the  perfect  animal,  and  the  record  is  much 
vitiated  by  the  struggle  for  existence  which  the  freely- 
living  larvre  have  to  undergo."  The  former  phenomenon, 
the  obliteration  of  the  ontogenetic  epitome,  is  effected  by 
the  law  of  simplified  or  abridged  heredity.  The  latter  phe- 
nomenon, the  vitiation  of  the  ontogenetic  epitome,  is  caused 
by  the  law  of  modified  or  vitiated  heredity.  In  accordance 
with  this  latter  law,  the  young  forms  of  animals  (not  only 
freely-living  larvae,  but  also  embryos  enclosed  in  the  mother's 
body)  may  be  modified  by  the  influence  of  the  immediate 
surroundings,  just  as  fully  formed  animals  are  modified  by 
adaptation  to  the  external  conditions  of  their  existence; 
the  very  species  are  sometimes  modified  during  germination. 
In  accordance  with  the  law  of  shortened  heredity,  it  is 
advantageous  to  all  higher  organisms  (and  the  more  so  the 
higher  their  development)  that  the  original  course  of 
development  should  be  shortened  and  simplified,  and, 
consequently,  that  the  ancestral  traditions  should  be 
obliterated.  The  higher  the  individual  organism  stands  in 
the  animal  kingdom,  the  less  completely  does  it  reproduce, 
in  its  Ontogeny,  the  entire  series  of  its  ancestors,  for 
reasons  which  are  partly  known,  partly  yet  undiscovered 


PALINGENESIS  AND   KENOGENESIS.  409 

The  fact  is  simply  shown  by  a  comparison  of  the  various 
histories  of  individual  evolution  of  higher  and  lower  animals 
of  the  same  tribe.111 

In  order  to  give  its  due  weight  to  this  significant 
relation,  we  have  classed  the  whole  series  of  ontogenetic 
phenomena,  of  the  phenomena  occuring  in  the  evolution  of 
an  individual,  in  two  different  groups,  placing  the  palin- 
genetic  phenomena  in  one  group,  the  kenogenetic  in  the 
other.  To  Palingenesis,  or  inherited  evolution,  we  referred 
those  incidents  in  germ-history  which  may  be  regarded  as 
accurately  inherited  from  the  history  of  the  tribe.  On  the 
other  hand,  we  applied  the  term  Kenogenesis,  or  vitiated 
evolution,  to  such  ontogenetic  processes  as  were  not 
directly  referable  to  corresponding  phylogenetic  incidents, 
but  were,  on  the  contrary,  to  be  explained  as  modifications, 
or  vitiations,  of  the  latter.  In  consequence  of  this  critical 
separation  of  palingenetic  from  kenogenetic  germinal 
phenomena,  the  fundamental  law  of  Biogeny  was  more 
accurately  defined  as  follows :  The  short  and  quick  history 
of  the  germ  (Ontogeny)  is  a  compressed  epitome  of  the 
long  and  slow  history  of  the  tribe;  this  epitome  is  the 
more  correct  and  complete,  in  proportion  as  the  inherited  or 
epitomized  evolution  (Palingenesis)  is  retained  by  heredity, 
and  the  less  vitiated  evolution  (Kenogenesis)  is  introduced 
by  adaptation.10 

In  order  correctly  to  distinguish  the  palingenetic  from 
the  kenogenetic  phenomena  of  germ-history,  and  from  these 
rightly  to  infer  the  tribal  history,  we  must  especially  apply 
ourselves  to  a  comparative  study  of  Ontogeny.  It  is  only 
by  comparing  the  germ-history  of  allied  forms  that  we  are 
able  to  discover  the  traces  of  their  tribal  history.  For  this 


4- tO  THE   EVOLUTION    OF   MAN. 

purpose  we  may  most  advantageously  apply  the  method 
which  geologists  have  long  used  in  determining  the  order 
of  the  sedimentary  rocks  in  the  crust  of  the  earth.  Most 
people  know  that  the  solid  crust  of  our  globe,  a  thin  shell 
which  surrounds  the  glowing  and  fluid  main  mass  in  its 
interior,  consists  of  two  chief  classes  of  rocks :  firstly,  the 
so-called  Volcanic,  or  Plutonic  rocks,  produced  directly  by 
the  solidification  of  the  molten  internal  mass  of  the  earth 
upon  the  surface ;  and,  secondly,  the  so-called  Neptunian,  or 
Sedimentary  rocks,  produced  from  the  former  by  the  trans- 
forming agency  of  water,  and  deposited,  in  stratified  layers, 
under  water.  At  first,  each  of  these  Neptunian  layers 
formed  a  stratum  of  soft  mud ;  but  in  the  course  of 
thousands  of  years  they  solidified  into  firm,  hard  masses  of 
rock  (sandstone,  marl,  chalk,  etc.),  at  the  same  time  perma- 
nently enclosing  in  their  own  mass  such  hard  and  imperish- 
able bodies  as  had  found  their  way  into  the  soft  mud. 
Among  the  bodies,  which  were  in  this  way  either  actually 
fossilized,  or  left  the  characteristic  imprints  of  their  forms 
in  the  soft  clay,  the  harder  parts  of  the  animals  and  plants 
which  lived  and  died  on  the  spot  during  the  stratification  of 
mud  are  especially  frequent. 

Each  Neptunian  rock-stratum  contains  its  own  charac- 
teristic fossils — the  remains  of  such  animals  and  plants  as 
lived  during  that  particular  epoch  of  the  earth's  history. 
By  comparing  these  strata,  it  is  possible  to  review  the  whole 
connected  series  of  earth-periods.  All  geologists  are  now 
agreed  that  such  a  positive,  historical  series  of  rock  forma- 
tions is  demonstrable,  and  that  the  lowest  of  these  strata 
were  deposited  in  primaeval  times,  the  upper  in  the  most 
recent  times.  But  in  no  one  place  on  the  surface  of  the- 


THE   GEOLOGICAL   METHOD.  411 

globe  is  the  entire  series  of  the  strata-system  perfect,  with 
layer  on  layer  in  due  succession ;  in  no  place  is  the  series  even 
approximately  complete.  In  fact,  the  order  of  the  different 
strata  of  the  earth  and  of  the  corresponding  periods  of  the 
earth's  history,  as  commonly  conceived  by  geologists,  is  only 
hypothetical,  and  does  not  actually  exist ;  it  is  the  result  of 
the  comparison  of  a  number  of  separate  observations  of  the 
sequence  of  strata  at  various  points  on  the  earth's  surface. 

We  shall  treat  the  Phylogeny  of  Man  in  a  similar  way. 
We  will  endeavour  to  form  the  various  phylogenetic  frag- 
ments, occurring  in  very  different  groups  of  the  animal 
kingdom,  into  an  approximately  correct  representation  of 
the  ancestral  line  of  Man.  We  shall  find,  that  it  is  really 
possible,  by  rightly  grouping  and  comparing  the  germ-history 
of  very  diverse  animals,  to  obtain  an  approximately  perfect 
picture  of  the  palseontological  development  of  the  ancestors 
of  Man  and  of  Mammals ;  a  picture,  such  as  could  never  be 
formed  from  the  Ontogeny  of  the  Mammals.  In  consequence 
of  the  kenogenetic  processes  to  which  we  have  alluded,  in 
consequence  of  vitiated  and  of  abridged  heredity,  whole 
series  of  the  lower  stages  of  evolution,  especially  in  the  most 
ancient  periods,  have  fallen  out  from  the  germ-history  of 
Man  and  of  other  Mammals,  or  have  been  vitiated  by  modifi- 
cation. But  in  the  lower  Vertebrates  and  in  their  Inverte- 
brate ancestors  we  meet  with  these  very  low  form-stages 
in  all  their  original  purity.  Especially  in  the  lowest  of  all 
Vertebrates,  the  Amphioxus,  the  most  ancient  ancestral  forms 
have  been  perfectly  retained  in  the  evolution  of  the  germ. 
So,  too,  we  find  strong  evidence  in  the  Fishes,  which  stand 
midway  between  the  lower  and  higher  Vertebrates,  and 
which  explain  several  other  phylogenetic  periods.  Lastly 
29 


412  THE   EVOLUTION   OF   MAN 

come  the  highest  Vertebrates,  in  which  the  middle  and  the 
older  stages  of  ancestral  evolution  have  been  either  falsified 
or  abridged,  but  in  which  the  later  stages  of  the  phylo- 
genetic  process  are  still  well  retained  in  the  Ontogeny. 
Thus  it  is  possible,  by  collating  and  comparing  the  history 
of  individual  development  in  the  different  groups  of  Verte- 
brates, to  obtain  an  approximately  complete  picture  of  the 
palaeontological  history  of  the  development  of  the  ancestors 
of  Man,  within  the  vertebrate  tribe.  If  we  descend  below 
the  lowest  Vertebrates,  and  compare  the  germ -history  of 
these  with  that  of  the  phylogenetically  allied  Invertebrates, 
we  can  trace  the  genealogical  line  of  our  animal  ancestors 
much  further,  as  far  back  as  the  lowest  Plant-animals 
(Zoophytes)  and  Primitive-animals  (Protozoa). 

In  now  treading  the  obscure  path  of  this  phylogenetic 
labyrinth,  holding  fast  the  Ariadne's  clew  of  the  funda- 
mental law  of  Biogeny  and  guided  by  the  light  of  Com- 
parative Anatomy,  we  must,  in  accordance  with  the  method 
we  have  just  indicated,  search  out  from  among  the  diverse 
germ-histories  of  very  different  animals,  those  fragments  from 
which  we  may  construct  the  tribal  history  of  Man ;  and  we 
must  arrange  these  fragments  in  their  proper  order.  Here 
again  I  would  call  special  attention  to  the  fact  that  we 
employ  this  method  with  the  same  certainty  and  with  the 
same  right  as  do  geologists.  No  geologist  has  seen  the 
actual  process  in-  which  the  gigantic  rock-masses,  composing 
the  Carboniferous  formations,  the  Jurassic,  the  Cretaceous, 
etc.,  were  actually  deposited  by  the  water.  Nor  lias  any 
geologist  actually  seen  that  these  various  sedimentary  rocky 
formations  originated  in  a  particular  sequence ;  and  yet  all 
agree  as  to  this  sequence.  The  reason  of  this  is  that  only 


THE   LANCELET   AND  /THE   ASCIDIAN.  413 

on  the  hypothesis  of  this  sedimentary  stratification  and  of 
this  sequence,  is  the  nature  and  origin  of  these  rock-masses 
intelligible.  Since  they  are  only  conceivable  and  explicable 
by  these  "  geological  hypotheses,"  these  hypotheses  are 
universally  accepted  as  "  geological  theories." 

On  similar  grounds,  our  phylogenetic  hypotheses  can 
claim  precisely  the  same  force.  In  proposing  them  we 
follow  the  same  inductive  and  deductive  methods,  and  with 
the  same  approximate  certainty,  as  are  followed  by  geolo- 
gists; because  only  with  the  aid  of  these  phylogenetic 
hypotheses  is  the  nature  and  origin  of  Man  and  of  other 
organisms  conceivable  ;  and  because  these  hypotheses  only 
can  satisfy  our  reason  in  its  demand  for  causality,  therefore 
we  hold  these  to  be  just ;  therefore  we  claim  for  them  the 
rank  of  "  biological  theories."  And,  just  as  geological 
hypotheses,  which  even  in  the  beginning  of  the  present 
century  were  derided  as  speculative  castles  in  the  air,  are 
now  universally  accepted ;  so,  too,  before  the  close  of  this 
century  will  our  phylogenetic  hypotheses  be  received  as 
valid,  although  they  are  at  present  ridiculed  by  the  narrow- 
minded  majority  of  naturalists  as  "the  dreams  of  the 
physio-philosophers."  It  is  true,  our  task,  as  we  shall  find, 
is  not  so  simple  as  that  of  the  geologists.  It  surpasses  the 
latter  in  difficulty  and  complexity  in  the  same  proportion  as 
the  organization  of  Man  is  higher  than  the  structure  of  the 
rock.112 

When  we  approach  our  task,  we  obtain  very  essential 
aid  by  first  closely  studying  the  comparative  germ-history 
of  two  low  animal  forms.  One  of  these  is  the  Lancelet 
(Amphioxus),  and  the  other  is  the  Sea-squirt  (Astidia) 
(Plates  X.  and  XL).  Both  animals  are  extremely  significant 


414  THE   EVOLUTION   OF  MAN. 

Both  stand  on  the  borderland  between  the  two  chief  divisions 
of  the  animal  kingdom,  which  since  the  time  of  Lamarck 
(1801)  have  been  distinguished  as  the  Vertebrates  and 
the  Invertebrates.  The  Vertebrates  embrace  the  already 
mentioned  classes  from  the  Lancelet  up  to  Man  (Acephala, 
Lampreys,  Fishes,  Double-breathers,  or  Dipneusta,  Amphibia, 
Reptiles,  Birds,  Mammals).  In  contradistinction  to  these,  all 
other  animals  have  usually,  in  agreement  with  the  example 
of  Lamarck,  been  classed  as  "  Invertebrates."  But,  as  we 
have  already  had  occasion  to  remark,  the  Invertebrates  in 
turn  consist  of  several  quite  distinct  tribes.  Of  these,  the 
Star-animals  (Echinoderma),  the  Soft-bodied  Animals  (Mol- 
lusca),  and  the  Articulated  Animals  (Arthropoda),  do  not 
interest  us  here,  because  they  are  independent  main  branches 
of  the  animal  genealogical  tree,  which  are  quite  distinct 
from  the  Vertebrates.  The  class  of  Worms  is,  on  the 
other  hand,  extremely  interesting  to  us.  In  this  group 
a  very  remarkable  class  of  animals  exists  which  has  only 
recently  been  carefully  studied,  and  which  bears  most 
significantly  on  the  genealogical  tree  of  Vertebrates.  This 
class  is  that  of  the  Mantle-animals  (Tunicata).  One 
member  of  this  class,  the  Sea-squirts  (Ascidia),  very  closely 
resembles  in  its  internal  structure  and  in  its  germination 
the  lowest  Vertebrate,  the  Lancelet  (Amphioxus).  Till  a 
few  years  ago  no  one  suspected  the  close  connection  be- 
tween these  two  apparently  quite  different  animal  forms, 
and  it  was  a  very  lucky  accident  that  just  now,  while  the 
question  as  to  the  descent  of  the  Vertebrates  from  Inverte- 
brates is  foremost,  the  germ-history  of  these  two  mosi 
closely  allied  animals  was  discovered.  In  order  rightly  tc 
understand  the  germ-history  of  the  Lancelet  and  the  Sea> 


HISTORY  OF  THE  LANCELET  415 

squirt,  we  must  first  consider  these  two  remarkable  animals 
in  their  perfect  state  and  compare  their  anatomies. 

We  begin  with  the  Lancelot,  or  Amphioxus,  which,  after 
Man,  is  the  most  important  and  interesting  of  all  Vertebrates. 
(Of.  Fig.  151,  and  Plate  XI.  Fig.  15.)  The  Lancelet  was  first 
described  in  1778  by  a  German  naturalist,  named  Pallas. 
He  received  this  little  animal  from  the  British  North  Sea, 
and,  thinking  that  in  this  animal  he  recognized  a  form 
closely  allied  to  the  common  Naked  Snail  (Limax),  he  gave 
it  the  name  of  Limax  lanceolatus.  For  more  than  half  a 
centuiy,  no  one  troubled  himself  about  this  reputed  Naked 
Snail.  Not  till  1834  was  this  insignificant  creature  observed 
alive  in  the  sand  at  Naples,  by  a  local  zoologist  named 
Costa.  He  asserted  that  it  was  no  snail,  but  a  diminutive 
fish,  and  gave  it  the  name  of  Branchiostoma  lubricum.  Just 
about  the  same  time  the  English  naturalist,  Yarrell,  showed 
that  it  possessed  an  internal  axial  skeleton,  and  called  the 
animal  Amphioxus  lanceolatus.  Then,  in  1839,  it  was 
studied  most  closely  by  Johannes  Miiller  of  Berlin,  to 
whom  we  are  indebted  for  a  very  profound  and  thorough 
dissertation  upon  its  anatomy.113  Recently  our  knowledge 
of  the  animal  has  been  greatly  extended,  and  its  more 
delicate  structure  especially  has  become  better  known.114 

The  Amphioxus  lives  in  flat,  sandy  localities  on  the  sea- 
coast,  partly  buried  in  the  sand,  and  appears  to  be  very 
widely  distributed  in  various  seas.  It  is  found  in  the 
North  Sea  (on  the  British  and  Scandinavian  coasts,  and 
also  in  Heligoland),  in  various  parts  of  the  Mediterranean 
(e.g.,  at  Nice,  Naples,  Messina).  It  also  occurs  on  the  coast 
of  Brazil  and  on  the  distant  shores  of  the  Pacific  Ocean 
(the  coast  of  Peru,  Borneo,  China,  etc.).  Everywhere  this 
remarkable  little  animal  appears  in  the  same  simple  form.68 


4-1 6  THE   EVOLUTION   OF   MAN. 

Johannes  Hiiller  referred  the  Lancelot  to  the  class  of 
Fishes,  though  he  insisted  that  the  differences  between  this 
lowest  of  the  Vertebrates  and  the  lowest  Fishes  are  much 
more  considerable  than  the  difference  between  all  Fishes  and 
the  Amphibia.  But  this  is  far  from  expressing  the  real 
significance  of  this  important  little  animal.  Indeed,  we 
might  confidently  lay  down  the  proposition  that  the  dif- 
ference between  the  Amphioxus  and  the  Fishes  is  far  greater 
than  between  the  Fishes  and  Man  and  all  other  Vertebrates. 
Nay,  so  widely  does  the  Amphioxus  differ  in  its  whole 
organization  from  the  rest  of  the  Vertebrates,  that,  according 
to  the  laws  of  systematic  logic,  we  are  forced  to  distinguish 
two  main  divisions  of  the  vertebrate  tribe:  (1)  the  Skull-less 
Animals,  or  Acrania  (the  Amphioxus  and  the  extinct  allied 
forms) ;  and  (2)  the  Skulled  Animals,  or  Craniota  (Man  and 
aU  other  Vertebrates.)115 

The  first  and  lower  division  consists  of  Vertebrates 
without  head,  brain,  or  skull,  for  which  reason  they  are 
called  Skull-less  Animals,  or  Acrania.  Of  these,  the  only 
extant  representative  is  the  Amphioxus,  though  in  the 
earlier  periods  of  the  earth's  history  very  numerous  and 
varied  forms  belonging  to  this  division  must  have  existed. 
We  may  here  lay  down  a  universal  law,  which  must  be 
accepted  by  every  adherent  of  the  theory  of  evolution :  viz., 
such  entirely  peculiar  and  isolated  animal  forms,  as  the 
Amphioxus — which  apparently  stands  alone  in  the  whole 
system  of  animals — are  always  the  last  survivors  of  an 
extinct  group,  numerous  and  diversified  forms  of  which 
existed  at  an  earlier  period.  As  the  whole  Amphioxus  is 
soft,  and  has  no  firm  organs,  capable  of  being  fossilized,  we 
may  suppose  that  all  its  numerous  extinct  kindred  were 


ACRANIA  AND   CRANIOTA.  417 

equally  soft  and  were,  therefore,  equally  incapable  of  being 
petrified  and  of  leaving  any  fossil  impressions. 

Contrasted  with  the  Skull-less  Animals  stands  the  other 
main  division  of  Vertebrates,  embracing  all  the  rest  of  that 
class  from  Fishes  to  Man.  These  all  have  a  head,  clearly 
marked  from  the  trunk,  with  a  skull  and  brain;  they  all 
have  a  centralized  heart,  developed  kidneys,  etc.  They  are 
called  Skulled  Animals,  or  Craniota.  But  in  the  earliest 
stages  of  their  existence.even  these  are  skull-less.  As  we  have 
seen  in  the  Ontogeny  of  Man,  every  Mammal,  in  the  early 
stages  of  individual  development  passes  through  a  condition 
in  which  it  has  neither  head,  nor  skull,  nor  brain,  and 
possesses  only  the  well-known,  simple  form  of  a  lyre-shaped 
disc,  or  of  a  shoe-sole,  without  any  limbs  or  extremities. 
Comparing  these  early  embryonic  forms  with  the  developed 
Lancelet,  we  may  say,  that  the  Amphioxus  is  in  a  certain 
sense  a  persistent  embryo,  a  permanent  germ-form  of 
Skulled-animals ;  it  never  passes  beyond  a  certain  low, 
early  youthful  condition,  out  of  which  we  have  long  since 


The  perfectly  formed  Lancelet  (Fig.  151)  is  5  to  6  c.m.  in 
length  (above  two  inches),  is  either  colourless  or  slightly 
reddish,  and  is  shaped  like  a  narrow  lanceolate  leaf.  The 
body  is  pointed  at  both  ends,  and  much  compressed  later- 
ally. There  is  no  trace  of  limbs.  The  outer  skin-covering 
is  very  delicate  and  thin,  naked,  translucent,  and  consists  of 
two  distinct  strata;  a  simple  external  skin,  the  outer  skin 
(epidermis ;  Plate  X.  Fig.  13,  h],  and  a  fibrous  leather-skin 
(corium),  lying  below  the  epidermis  (Fig.  13,?).  The  central 
line  of  the  back  is  traversed  by  a  narrow  fin-like  ridge 
which  widens  behind  into  an  oval  tail -fin.  and  is  prolonged 


418  THE   EVOLUTION   OF   MAN. 

underneath  into  a  short,  anal  fin.  The  fin-like  riclge  is  sup- 
ported by  a  great  number  of  small  and  delicate  quadrangular 
plates  (Plate  XI.  15,  /).  The  delicate  parallel  lines  under 
the  skin,  which  describe  an  acute  angle  forward  along  the 
central  line  of  each  side,  are  the  boundary  lines  of  the 
numerous  dorsal  muscles  (Fig.  15,  r  and  6). 

In  the  centre  of  the  body  is  a  thin  cartilaginous 
cord,  which  traverses  the  longitudinal  axis  of  the  entire  body 
from  front  to  rear,  and  is  symmetrically  sharpened  at  both 
ends  (Fig.  151,  i}.  This  is  the  notochord  (chorda  dorsalis), 
which  in  this  case  takes  the  place  of  the  backbone,  or 
vertebral  column.  In  the  Amphioxus  the  notochord  does  not 
develop  further,  but  remains  permanently  in  this  most  simple 
original  condition.  It  is  enclosed  in  a  firm  membranous 
covering,  the  notochord-sheath.  The  nature  of  the  latter, 
and  of  the  formations  which  proceed  from  it,  may  be  best 
seen  in  the  transverse  section  of  the  Amphioxus  (Fig.  152  ; 
Plate  X.  Fig.  13,  cs).  Immediately  above  the  chorda  the 
notochord-sheath  forms  a  cylindrical  tube,  and  in  this  tube 
the  central  nervous  system  lies  enclosed,  the  spinal  or  me- 
dullary tube  (Plate  XI.  Fig.  15, 7?i).  This  important  mental 
organ  retains  throughout  life  this  most  simple  form,  that  of 
a  cylindrical  tube,  the  anterior  and  posterior  ends  of  which 
are  almost  equally  simple,  and  the  thick  wall  of  which 
encloses  a  narrow  canal.  The  anterior  end  is,  indeed,  rather 
rounder,  and  contains  a  small,  hardly  noticeable,  bladder- 
like  swelling  of  the  canal  (Fig.  15,  m-^).  This  may  be  re- 
garded as  the  first  indication  of  a  real  brain-bladder ;  as  a 
rudimentary  brain.  On  the  foremost  end  there  is  also  H 
little  black  pigment-spot,  the  rudiment  of  an  eye.  Near 
this  eye-spot,  on  the  left  side,  there  is  a  little  ciliated  groove, 


STRUCTURE   OF   THE   LANCELET.  419 

the  single  organ  of  smell.  The  organ  of  hearing  is  entirely 
wanting.  This  defective  evolution  of  the  higher  sense- 
organs  is  probably  in  great  measure  explicable  as  not 
original,  but  as  a  degeneration. 

Bqlow  the  notochord  runs  a  very  simple  intestinal  canal, 
a  tube,  which,  on  the  ventral  side  of  the  little  animal,  opens 
in  front  in  a  mouth,  and  at  the  back  in  an  anus.  The  mouth 
is  oval,  and  surrounded  by  a  cartilaginous  circle,  on  which 
are  20  to  30  filaments  of  cartilage  (organs  of  taste)  (Fig. 
151,  a).  By  a  contraction  in  the  centre,  the  intestinal  canal 
divides  in  the  centre  into  two  very  different  parts,  of  about 
equal  length.  The  anterior  division  acts  as  a  respiratory 
organ,  the  posterior  end  as  a  digestive  organ.  The  anterior 
half  forms  a  wide  gill-body,  the  lattice-like  wall  of  which 
is  pierced  by  numerous  gill-openings  (Fig.  151,  d,  and  Plate 
XI.  Fig.  15,  k}.  The  delicate  bars  of  the  gill-body,  between  the 
openings,  are  supported  by  small,  firm  parallel  staves,  which 
are  connected  together  in  pairs  by  cross-staves.  The  water 
which  the  Amphioxus  takes  in  through  its  mouth  passes 
through  these  openings  in  the  gill-body  into  the  large  gill- 
cavity  which  suiTound  the  gill-body,  and  then  passes  further 
back  and  out  through  the  breath-hole,  or  gill-pore  (jporus 
branchialis ;  Fig.  151,  c).  On  the  ventral  side  of  the  gill- 
body  there  is,  along  the  central  line,  a  ciliated  groove 
(the  hypobranchial  groove),  which  also  occurs  in  Ascidians 
and  in  the  larvae  of  Cyclostomi ;  it  is  of  interest  because 
from  it  in  the  higher  Vertebrates  is  developed  the  thyroid 
cartilage  on  the  throat  (on  the  lower  part  of  the  so-called 
Adam's  apple;  Fig.  15,  y). 

Behind  the  breathing,  or  respiratory  part  of  the  intestinal 
canal  comes,  secondly,  the  digestive  part.  The  small  bodies 


420- 


THE   EVOLUTION    OF   MAX. 


which  the  Amphioxus  takes  up  in  the  water  it  breathes — 
Infusoria,  Diatomacese,  parts  of  decayed  plants,  and  animal 

FIG.  151. — Lancelet  (Amphioxus  lanceolatus) ,  twice 
the  natural  size ;  seen  from  the  left  side  (the  longitu- 
dinal axis  stands  upright ;  the  month  end  is  turned 
upwards,  the  tail  end  downwards,  as  in  Plate  XI. 
Fig.  15) :  a,  month-opening,  surrounded  by  hairs ; 
I,  anal  opening;  c,  gill-pore  (porus  branchialis)  ; 
d,  gill-body ;  e,  stomach ;  /,  liver  ;  y,  small  intestine  ; 
h,  gill-cavity;  i,  notochord  (below  this  the  aorta); 
k,  aorta-arch ;  I,  main  trunk  of  the  gill-artery ;  m, 
swellings  on  the  branches  of  the  latter;  n,  hollow 
vein  (vena  cava) ;  o,  intestinal  vein. 

bodies,  etc. — pass  back  from  the  gill-body 
into  the  digestive  section  of  the  intes- 
tinal canal,  and  are  there  taken  up  as 
food  and  assimilated.  From  a  rather  wider 
section,  corresponding  to  the  stomach 
(Fig.  151,  e),  proceeds  an  oblong,  pouch- 
like  blind-sac  (/),  which  passes  directly 
forward,  and  ends  on  the  right  side  of  the 
gill-body.  This  is  the  liver  of  the  Amphi- 
oxus, the  simplest  form  of  liver  that  we 
know  of  in  any  Vertebrate.  In  Man  also, 
as  we  shall  see,  the  liver  develops  as  a 
pouch-shaped  blind-sac,  which  protrudes 
from  the  intestinal  canal  behind  the 
stomach. 

The  structure  of  the  system  of  blood- 
vessels in  our  little  animal  is  not  less  re- 
markable than  that  of  the  intestine.  For 
while  all  other  Vertebrates  have  a  compressed,  thick,  purse- 
shaped  heart,  which  develops  at  the  throat  from  the  lower 


S"'\ 


CIRCULATION   IN   THE   LANCELET.  421 

wall  of  the  anterior  intestine,  and  from  which  the  blood- 
vessels proceed,  there  is  in  the  Amphioxus  no  special  central- 
ized heart,  propelling  the  blood  by  its  pulsations.  Instead, 
the  movement  of  the  blood  in  the  Amphioxus,  as  in  the 
Ringed  Worms  (Annelida),  is  effected  by  the  thin  tubular 
blood-vessels  themselves,  which  perform  the  functions  of  the 
heart,  contracting  and  pulsating  through  their  entire  length, 
and  thus  driving  the  colourless  blood  through  the  whole 
body.  This  circulation  is  so  simple  and  yet  so  remarkable, 
that  we  will  briefly  consider  it.  Let  us  begin  in  front  at 
the  lower  side  of  the  gill-body.  In  the  central  line  of  this 
lies  a  large  main  vessel,  which  corresponds  to  the  heart  of 
other'  Vertebrates  and  to  the  main  gill-artery  proceeding 
from  its  heart,  and  which  propels  the  blood  into  the  gills 
(Fig.  151,  £).  The  anterior  portion  of  this  is -swollen  like 
a  heart  and  is  extended  (immediately  in  front  of  the  first 
gill-opening).  Numerous  little  arching  vessels  rise  on  each 
side  from  this  gill-artery,  form  little  heart-like  swellings 
(bulbs,  m)  at  their  point  of  departure,  traverse  the  gill- 
arches,  between  the  gill-openings,  round  the  anterior  intes- 
tine, and  unite  as  gill-veins  above  the  gill-body  in  a  great 
main  vessel,  which  passes  below  the  notochord.  This  vessel 
is  the  primitive  aorta  (Plate  X.  Fig.  13,  t ;  Plate  XL 
Fig.  15,  t}.  The  aorta  passes  between  the  intestine  and  the 
notochord  precisely  as  in  all  the  higher  Vertebrates.  The 
branch-vessels  which  this  aorta  sends  to  all  parts  of  the 
entire  body,  again  collect  into  a  large  venous  vessel,  which 
passes  to  the  lower  side  of  the  intestine,  and  which  may 
here  be  called  the  intestinal  vein  (Fig.  151,  o  ;  Plate  X. 
Fig.  15,  v ;  Plate  XL  Fig.  13,  v).  It  passes  on  further  over  the 
pouch-like  liver,  there  forms  a  kind  of  cystic  vein,  weaving 


422  THE   EVOLUTION   OF   MAN. 

a  fine  vascular  network  around  the  blind-sac  of  the 
and  then  passes,  as  a  liver  vein,  into  a  vessel,  directed 
toward  the  front,  which  we  may  call  the  hollow  vein 
(Fig.  151,  ri).  This  last  passes  again  directly  to  the  ventral 
side  of  the  gill-body,  and  here  directly  re-enters  the  gill- 
artery,  which  we  took  as  a  starting-point.  Like 'a  circular 
closed  aqueduct,  this  single  main  vascular  tube  passes  along 
the  intestinal  tube  through  the  whole  body  of  the  Amphi- 
oxus,  pulsating  throughout  its  entire  length  both  above  and 
below.  Within  about  a  minute ,  the  colourless  blood  is  thus 
driven  through  the  whole  body  of  the  little  creature.  When, 
in  pulsating,  the  upper  tube  contracts,  the  lower  fills  with 
blood,  and  vice  versd.  Above,  the  current  of  blood  is  from 
front  to  rear ;  below,  on  the  contrary,  it  is  from  the  rear  to 
the  front.  The  entire  long  vascular  tube,  which  runs  below 
along  the  ventral  side  of  the  intestinal  tube,  and  which 
contains  venous  blood,  probably  represents  the  so-called 
ventral  blood-vessel  of  Worms  (Plate  IV.  Fig.  7,  v).  On  the 
other  hand,  the  long  straight  vascular  tube,  which  runs 
above  along  the  dorsal  line  of  the  intestinal  tube,  between 
it  and  the  notochord,  and  which  contains  arterial  blood,  is, 
on  the  one  hand,  evidently  homologous  with  the  aorta  of 
other  Vertebrates,  and,  on  the  other  hand,  with  the  so-called 
dorsal  blood-vessel  of  Worms  (Plate  IV.  Fig.  7,0- 

Johannes  Muller  recognized  this  important  similarity  in 
the  formation  of  the  system  of  blood-vessels  of  the  Lancelot 
and  of  Worms.  He  directed  special  attention  to  the  analo- 
gies of  the  two,  and  their  physiological  resemblance,  the 
blood  in  both  being  driven  by  the  pulsating  contractions  of 
the  great  vascular  tubes  throughout  their  entire  length,  and 
not  by  a  centralized  heart,  as  in  all  other  Vertebrates.  But 


SIDE   CANALS   AND   BODY-CAVITY.  423 

we  conceive  that  this  important  resemblance  is  more  than 
a  mere  analogy.  It  has  the  deeper  significance  of  a  true 
homology,  and  rests  on  a  morphological  resemblance  of  the 
organs  compared.  Thus,  the  Amphioxus  shows  us  that  the 
aorta,  the  single  main  artery  of  Vertebrates,  running 
between  the  intestine  and  the  notochord,  represents  the 
dorsal  blood-vessel  of  Worms.  On  the  other  hand,  the  ven- 
tral blood-vessel  of  the  latter  is  retained  only  in  the  single 
intestinal  vein  passing  below  the  intestine  of  the  Amphioxus 
(and  its  anterior  continuation  ;  cystic  vein,  liver  vein, 
hollow  vein  (v.  cava),  gill-artery).  In  the  developed  body 
of  all  other  Vertebrates  this  intestinal  vein  (originally  the 
main  venous  blood-vessel !)  is  far  outstripped  by  other 
veins. 

Together  with  the  real  blood-vessels,  special  absorbing 
lymph-vessels  seem  to  exist  in  the  Amphioxus.  Several 
canals,  extending  under  the  skin,  have  recently  been 
regarded  in  this  light,  especially  the  narrow  "ventral  canals" 
(Fig.  152,  >S1),  and  wide  "side  canals"  (S).  Both  pass  along 
the  whole  length  of  the  ventral  side  and  contain  colourless 
lymph.  The  side  canals  ($)  must  possibly  be  regarded  as 
the  last  remnants  of  degenerated  primitive  kidney  ducts. 
They  lie  in  the  two  parallel  side  folds  of  the  ventral  skin 
( F),  ending  blindly  both  in  front  and  behind,  and  do  not 
open  outwards,  as  was  supposed  till  recently. 

The  real  body-cavity  (cosloma)  in  the  Amphioxus  (Fig. 
152,  Lh}  is  extraordinarily  narrow  and  small.  It  surrounds 
the  intestinal  tube  in  its  narrow  cavity,  and  is  probably  con- 
nected with  the  lymph  spaces.  Formerly  it  was  confused 
.with  the  large  respiratory  cavity  or  gill-cavity  (A),  which  is 
of  entirely  different  morphological  and  physiological  signifi- 


424 


THE   EVOLUTION    OF    MAN. 


FIG.  152. — Transverse  section  through  the  anterior  part  of  a  Lancelet. 
(After  Eolph.)  The  outer  covering  forms  the  single  cell-stratum  of  the  outer 
skin  (epidermis,  E).  Below  this  lies  the  thin  leather  skin  (cerium),  the 
inner  tissue  of  which  is  thickened  below  (U)  ;  partition  walls  of  connective 
tissue  pass  inward  from  it  between  the  muscles  (M  J  and  to  the  chorda- 
sheath  ;  N,  medullary  tube ;  cJi,  notochord ;  Lh,  body-cavity  (cceloma) ;  A, 
gill-cavity;  L,  upper  wall  of  the  latter;  Elt  inner  wall  of  the  same;  E2, 
outer  wall  of  the  same ;  Kst,  gill-rods ;  M,  ventral  muscles ;  R,  Raphe,  or 
seam  formed  by  the  coalescence  of  the  ventral  folds  (gill-roofs)  ;  G,  eexual 
glands. 


KIDNEYS.  425 

cance.  The  true  body-cavity  (Lli)  is  filled  with  lymph, 
its  inner  wall  being  clothed  by  the  intestinal-fibrous  layer, 
its  outer  wall  by  the  skin-fibrous  layer.  The  gill-cavity  (A) 
is,  on  the  contrary,  filled  with  water,  and  its  whole  wall  is 
clothed  by  the  skin-sensory  layer.  The  latter  envelopes  the 
outer  surface  of  the  two  large,  lateral  gill-roofs,  the  lateral 
processes  from  the  body- wall,  which  grow  together  below . 
round  the  original  ventral  side,  and  unite  in  the  central  line 
(in  the  ventral  seam  or  raphe,  Fig.  152,  .R). 

On  each  side  of  this  ventral  seam,  on  the  inner  surface  of 
the  gill-roofs,  directly  in  front  of  the  gill-pore  (porus 
branchialis),  and  over  the  ventral  muscles  (M)  and  between 
the  sexual  glands  (GT),  lie  the  kidneys  of  the  Amphioxus. 
These  urinary  glands  are  present  in  the  simplest  form,  as 
glandular  epithelial  swellings  of  the  skin-sensory  layer. 
The  epithelial  cells  of  these  are  distinguished  by  peculiar 
size  and  nature,  and  contain  crystalline  deposits.  As  we 
regard  the  primitive  kidneys  of  other  Vertebrates  also  as 
originally  skin-glands,  and  as  we  derive  them  from  the  skin- 
sensory  layer,  it  is  very  interesting  to  find  these  organs 
permanently  retained  in  the  Lancelot  as  skin-glands. 

The  sexual  organs  also  appear  in  a  perfectly  simple 
form.  On  both  sides  of  the  gill-intestine,  in  the  central  part 
of  the  gill-cavity,  lie  from  twenty  to  thirty  small  elliptical  or 
roundly  four-cornered  sacs,  which  can  easily  be  seen  by  the 
naked  eye  from  without,  through  the  thin  transparent  wall 
of  the  body.  In  the  female,  these  little  sacs  are  the  ovaries, 
and  contain  numbers  of  simple  egg-cells  (Plate  X.  Fig.  13,  e). 
In  the  male,  these  are  replaced  by  the  testes,  heaps  of 
smaller  cells,  which  change  into  movable  whip-cells  (sperm- 
Both  kinds  of  sacs  lie  within  on  the  inner  wall  of 


426  THE    EVOLUTION    OF    MAN. 

the  gill-cavity,  and  have  no  special  channels  of  exit.  When 
the  eggs  of  the  female  and  the  seed  masses  of  the  male  are 
matured,  they  fall  into  the  body-cavity,  and  are  expelled 
through  the  gill-pore  (p.  branchialis). 

Now  on  trying  to  comprehend  in  one  connected  view  the 
results  of  our  anatomic  study  of  the  Amphioxus,  and  com  • 
paring  this  conception  with  the  known  organism  of  Man, 
the  contrast  between  the  two  seems  immense.  In  fact,  the 
most  perfect  vertebrate  organism,  represented  by  Man,  is  in 
every  respect  so  far  above  that  lowest  stage  in  which  the 
Lancelot  remains,  that  it  seems  at  first  almost  impossible  to 
place  both  organisms  in  the  same  main  division  of  the 
animal  kingdom.  And  yet  this  classification  is  based  on 
unassailable  grounds.  For  Man  represents  only  a  further 
advance  of  the  same  vertebrate  type,  which  in  all  its  rudi- 
mentary characters  is  unmistakably  seen  in  the  Amphioxus. 
It  is  only  necessary  to  recall  the  representation  which  has 
been  given  of  the  ideal  form  of  the  Primitive  Vertebrate 
(p.  256)  and  to  compare  wit'i  it  the  various  lower  stages  of 
development  of  the  human  embryo,  in  order  to  become 
convinced  of  our  near  relationship  to  the  Lancelet. 

It  is  true  that  a  few  zoologists  have  recently  maintained 
the  paradoxical  view  that  the  Amphioxus  is  in  no  way 
allied  to  Vertebrates.  This  was  asserted  especially  by  Karl 
Semper  and  Hobby  Kossman,  the  same  learned  pair  who 
discovered  in  Goethe  a  narrow-minded  upholder  of  the* 
constancy  of  species  (see  p.  91).  But  these  gentlemen  can 
only  have  uttered  this  assertion  in  order,  in  the  absence  of 
positive  merits,  to  make  their  names  known  by  negative 
instances.  One  who  at  the  present  time  maintains  that 
the  Amphioxus  is  not  allied  to  Vertebrates  goes  back  a 


THE  "ROUND-MOUTHS."  427 

whole  century,  even  beyond  Pallas  (1778),  and  only  proves 
that  his  notions  of  Comparative  Anatomy  and  of  the  history 
of  evolution  are  extremely  weak. 

The  Amphioxus  does,  indeed,  stand  very  far  below  all 
other. extant  Vertebrates.  It  is,  indeed,  without  the  head 
containing  a  developed  brain  and  skull,  which  distinguishes 
all  other  Vertebrates.  It  is  without  an  organ  of  hearing, 
and  without  a  centralized  heart,  such  as  all  others  possess ; 
perfect  kidneys  are  also  lacking.  Each  organ  appears  in  a 
simpler  and  more  imperfect  form  than  in  any  other  Ver- 
tebrate. And  yet,  the  rudimentary  characters,  the  connec- 
tion and  relative  position  of  all  the  organs,  are  the  same  as 
in  all  other  Vertebrates :  moreover,  they  all,  during  their 
embryonic  development,  pass,  at  an  early  period,  through  a 
stage  in  which  their  whole  organization  is  not  superior  to 
that  of  the  Amphioxus,  but  rather,  agrees  with  it  in  all 
essential  particulars.  (Of.  Table  IX.) 

In  order  to  be  thoroughly  convinced  of  this  important 
fact,  it  is  specially  instructive  to  compare  the  Amphioxus 
with  the  early  forms  of  development  of  those  Vertebrates 
which  are  most  nearly  allied  to  it  in  the  natural  system 
of  this  tribe.  This  is  the  class  of  the  Round-Mouths 
(Cyclostomi).  This  remarkable  class,  which  formerly  com- 
prehended many  species,  contains  at  the  present  day  but 
very  few  species,  which  are  separable  into  two  different 
groups.  One  group  is  formed  by  the  Hags  (Myxinoidce'), 
which  have  been  made  known  to  us  by  Johannes  Miiller's 
classic  work,  "  Vergleichende  Anatomie  der  Myxinoiden." 
The  other  group  is  formed  by  the  well-known  Lampreys,  or 
Rock-Suckers  (Petromyzonta),  which  are  eaten  as  a  delicacy. 
All  these  Round-Mouths  are  usually  included  in  the  class  of 

30 


4-28  THE   EVOLUTION   OF   MAN. 

Fishes.  They  stand,  however,  far  below  the  true  Fishes,  and 
form  a  very  interesting  connecting  group  between  them 
and  the  Lancelet.  How  near  they  stand  to  the  latter,  is 
clearly  seen  if  an  immature  Lamprey  (Petromyzon,  Plate  XL 
Fig.  16)  is  compared  with  the  Amphioxus  (Fig.  15).  In 
both,  the  notochord  (di)  is  in  the  same  simple  form,  as  is 
also  the  medullary  tube  (m),  lying  above  the  notochord,  and 
the  intestinal  tube  (d},  lying  below  the  notochord.  But  in 
the  Lamprey,  the  medullary  tube  soon  swells  in  front  into 
a  simple  pear-shaped  brain-bladder  (m^,  and  on  each  side 
of  this  appears  a  very  simple  eye  (an)  and  a  simple  ear- 
vesicle  (#).  The  nose  (n)  is  still  a  single  pit,  as  in  the 
Amphioxus.  The  two  sections  of  the  intestine  also,  the 
anterior  gill-intestine  (&)  and  the  posterior  stomach-intes- 
tine (d),  are  very  simple  in  the  Lamprey,  and  very  like 
those  of  the  Amphioxus.  On  the  other  hand,  there  is 
decided  progress  in  the  organization  of  the  heart,  which 
appears  below  the  gills  as  a  centralized  muscular  pouch,  and 
separates  into  an  auricle  (hv)  and  a  ventricle  (A&).  At  a 
later  period,  the  Lamprey  attains  to  a  considerably  higher 
state  of  development,  acquires  a  skull,  five  brain-bladders, 
a  series  of  independent  gill-pouches,  etc.  But  this  makes 
the  remarkable  similarity  of  its  young  larva  to  the  de- 
veloped Amphioxus  all  the  more  interesting.116 

The  Amphioxus,  which  is  thus  directly  connected,  on 
the  one  side,  with  the  Fishes  through  the  Round-Mouths 
(Cyclostomi),  and  thereby  to  the  series  of  higher  Vertebrates, 
is,  on  the  other  hand,  very  nearly  allied  to  a  lower  inver- 
tebrate sea-animal,  from  which,  at  first  sight,  it  seems  very 
far  removed.  This  remarkable  animal  is  the  Sea-squirt,  or 
Ascidian,  which  until  very  recently  was  regarded  as  being 


THE   SEA-SQUIRT,  OR  ASCIDIAN.  429 

nearly  related  to  the  Mussels,  and  was  therefore  classed 
with  the  Soft-bodied  Animals  (Mollusca).  But  since  1866, 
when  the  remarkable  germ-history  of  these  animals  was 
first  understood,  there  has  been  no  doubt  that  they  are 
unconnected  with  the  Soft-bodied  Animals.  On  the  con- 
trary, greatly  to  the  surprise  of  zoologists,  the  entire  mode 
of  their  individual  development  indicates  that  they  are  the 
nearest  allies  of  the  Vertebrates.  In  their  matured  con- 
dition the  Ascidians  are  shapeless  lumps,  which  at  first 
sight  certainly  do  not  look  like  animals.  The  oblong  body, 
often  rough,  or  covered  with  uneven  knobs,  in  which  no 
definite  outward  organs  are  distinguishable,  adheres  firmly 
by  one  end  to  sea-weeds,  stones,  or  to  the  bottom  of  the 
ocean.  Some  species  resemble  potatoes,  others  dried  plums. 
Many  Sea-squirts  form  very  insignificant  incrustations  on 
the  surface  of  stones  and  plants.  Some  of  the  larger  kinds 
are  eaten  like  oysters.  Fishermen,  who  know  them  well, 
regard  them  not  as  animals,  but  as  sea- weeds.  They  are 
frequently  offered  for  sale  together  with  other  low  sea- 
animals,  in  the  fish-markets  of  many  Italian  seaside  towns, 
under  the  name  of  Sea-fruit  (frutti  di  mare).  There  is 
indeed  nothing  outwardly  indicating  an  animal.  When 
they  are  drawn  from  the  sea  in  a  drag-net,  all  that  is 
noticeable  is  that  they  feebly  contract  their  bodies,  thus 
producing  a  spirting  of  water  from  certain  parts.  Most  of 
the  Sea-squirts  are  very  small,  only  a  few  lines,  or  at  most 
a  few  inches  long ;  a  few  species  attain  the  length  of  a  foot 
or  rather  more.  There  are  a  great  many  species,  which  are 
to  be  found  in  all  seas.  We  find  no  fossil  remains  'of  this 
class  of  animals,  because  they  have  no  hard  parts  capable 
of  petrifaction ;  but  they  are  certainly  of  very  great  an- 


430  THE   EVOLUTION   OF   MAN. 

tiquity,   and    must    have    existed    during    the    primeval 


The  whole  class  to  which  the  Ascidians  belong  bears  the 
name  of  Mantle-animals  (Tunicata),  because  the  body  is 
enclosed  in  a  thick  and  firm  membrane,  as  in  a  mantle  or 
tunic.  This  tunic,  which  is  sometimes  soft  and  jelly-like, 
sometimes  tough  and  leather-like,  sometimes  firm  and 
cartilaginous,  is  distinguished  by  many  remarkable  charac- 
teristics. Probably  the  most  remarkable  of  these  is,  that  it 
consists  of  a  woody  mass  or  cellulose,  the  same  plant-cell 
material  which  forms  the  firm  exterior  of  the  cells  of 
plants,  the  substance  of  the  wood.  The  Mantle-animals  are 
the  only  class  of  animals  which  really  possess  a  cellulose 
covering,  a  wood-like  envelope.  Sometimes  the  cellulose 
tunic  is  variegated,  at  other  times  it  is  colourless.  Not 
uncommonly  it  is  set  with  spines  or  hairs,  like  a  cactus. 
Many  foreign  substances,  such  as  stones,  sand,  fragments  of 
mussel-shells,  and  so  forth,  are  often  embedded  in  the  tunic. 
The  Sea-squirt  has,  therefore,  received  the  name  "  micro- 
cosm."117 

In  order  correctly  to  understand  the  internal  organiza- 
tion of  the  Sea-squirt,  and  thoroughly  to  compare  it  with  the 
Amphioxus,  we  must  place  ourselves  in  the  same  position  to 
it  as  to  the  latter  (Plate  XI.  Fig.  14,  on  the  left  side ;  the 
mouth  extremity  is  turned  upward,  the  back  to  the  right, 
the  abdomen  to  the  left).  The  posterior  end,  corresponding 
to  the  tail  of  the  Amphioxus,  is  usually  adherent,  often  by 
means  of  root-like  processes.  The  ventral  and  dorsal  sides 
are  internally  very  different,  but  are  often  externally  undis- 
tinguishable.  On  opening  the  thick  tunic,  in  order  to  note 
the  internal  organization,  we  observe  first  a  very  consider- 


STRUCTURE    OF   ASCIDIAN. 


431 


able  cavity,  filled  with  water;  this  is  the  gill-cavity,  or 
respiratory  cavity  (Fig.  153,  d;  Plate  XI.  Fig.  14,  d}.  It 
is  also  called  the  mantle  or  tunic  cavity,  because  it  receives, 


FIG.  153. — Structure  of  an  Ascidian 
(viewed  from  the  left  side,  as  in  Plate 
XII.  Fig.  14) ;  the  dorsal  side  is  turned 
towards  the  right,  the  ventral  side  to- 
wards the  left,  the  mouth-opening  (o) 
upwards  ;  at  the  opposite,  tail  extremity, 
the  ascidian  is  firmly  attached  to  some 
substance  below.  The  gill-intestine 
(fcr),  which  is  pierced  by  many  open- 
ings, continues  below  as  the  stomach- 
intestine.  The  large  intestine  opens 
through  the  anus  (a)  into  the  gill- 
cavity  (c?),  from  which  the  excrement 
is  removed  with  the  inhaled  water 
through  the  mouth  of  the  tunic  (a')  ;  m, 
tunic.  (After  Gegenbaur.) 


not  only  the  water  for  respir- 
atory purposes,  but  also  ex- 
crement and  the  sexual  pro- 
ducts. The  greater  part  of  the 
respiratory  cavity  is  occupied 
by  the  latticed  gill-sac  (br). 
The  latter  is  in  its  whole  posi- 
tion and  constitution  so  like 
the  gill-body  of  the  Amphioxus,  that  many  years  ago, 
before  anything  was  known  of  the  real  relationship  of  the 
two  animals,  the  English  naturalist,  Goodsir,  called  attention 
to  this  striking  similarity.  In  the  Sea-squirts  also  the 
mouth-opening  (o)  leads  directly  into  this  gill-sac.  The 
water  breathed  in  passes  through  the  openings  of  the 


4.32  THE   EVOLUTION    OF   MAN. 

latticed  gill-sac  into  the  gill-cavity,  and  is  removed  from 
there  by  the  respiratory  pore  or  excretory  opening  (a').  A 
ciliated  groove  traverses  the  ventral  side  of  the  gill-sac,  the 
same  "hypo-branchial  groove  "  which  we  found  before  in  the 
Amphioxus  at  the  same  place  (Plate  XI.  Fig.  14,  y,  15,  y}. 
The  food  of  the  Sea-squirt,  like  that  of  the  Amphioxus,  con- 
sists of  small  organisms,  Infusoria,  Diatomacece,  parts  of 
dismembered  sea-weeds  and  sea-animals,  etc.  These  pass 
with  the  inhaled  water  into  the  gill-sac,  and  from  the  end  of 
this  into  the  digestive  part  of  the  intestinal  canal,  first  into 
an  extension  answering  to  a  stomach  (Fig.  14,  my).  The 
small  intestine  connected  with  it  usually  forms  a  loop, 
curving  around  toward  the  front,  and  opens  in  a  vent  (Fig. 
153,  a),  not  directly  out,  but  first  into  the  gill-cavity ;  from 
here  the  excrement  is  removed,  together  with  the  inhaled 
water  and  the  sexual  products,  through  the  common  ex- 
cretory opening  (a/).  The  latter  is  sometimes  called  gill- 
pore,  or  respiratory  pore  (porus  branchialis),  sometimes  the 
cloacal  opening  (Plate  XI.  Fig.  149).  In  many  Sea-squirts,  a 
glandular  mass,  representing  the  liver,  opens  into  the  kites- 
tine  (Fig.  14,  IV).  In  some,  there  is  another  gland  near  the 
liver,  which  is  supposed  to  be  the  kidney  (Fig.  14,  u).  The 
real  body-cavity  (cceloma),  which  is  filled  with  blood  and 
surrounds  the  stomach,  is  very  small  in  the  Ascidian,  as  in 
the  Amphioxus,  and  equally  in  both  cases  is  usually  con- 
fused with  the  gill-cavity,  which  is  filled  with  water. 

In  the  mature  Sea-squirt  there  is  no  trace  of  a  noto- 
chord,  an  inner  bony  axis.  This  adds  interest  to  the  fact, 
that  the  young  animal,  as  it  emerges  from  the  egg,  has  a 
notochord  (Plate  X.  Fig.  5,  ch\  above  which  lies  a  rudimen- 
tary medullary  tube  (Fig.  5,  m}.  In  the  mature  Sea-squirt, 


HEART.    OF   THE  ASCIDIAN.  433 

this  tube  is  entirely  shrivelled  up,  and  forms  a  little  knot  of 
nerves  lying  near  the  front  above  the  gill-sac  (Fig.  14,  m). 
It  answers  to  the  so-called  upper  throat-ganglion,  or  the 
"  brain  "  of  other  Worms.  Special  organs  of  sense  are  either 
entirely  wanting,  or  exist  in  the  very  simplest  form,  as 
eye-specks  and  taste  papillae,  which  surround  the  mouth 
(Fig.  14,  au,  eyes).  The  muscular  system  is  very  feebly, 
and  irregularly  developed.  Immediately  below  the  thin 
leather-skin  (corium)  with  which  it  is  intimately  connected, 
is  a  thin  pouch-shaped  muscular  membrane,  as  in  the  lower 
Worms.  On  the  other  hand,  the  Sea-squirt  has  a  cen- 
tralized heart,  and  appears  in  this  respect  to  be  more  highly 
organized  than  the  Amphioxus.  On  the  ventral  side  of 
the  intestine,  at  a  considerable  distance  behind  the  gill-sac, 
lies  a  spindle-shaped  heart  (Fig.  14,  kz).  It  permanently 
retains  that  same  simple  pouch-shaped  form  which  the 
rudimentary  heart  of  the  Vertebrate  possesses  for  a  very 
short  time.  (Cf.  the  heart  of  the  human  embryo,  Fig. 
144,  c,  p.  392.)  This  simple  heart  of  the  Ascidian,  how- 
ever, exhibits  a  remarkable  peculiarity.  It  contracts  in 
alternate  directions.  While  in  all  other  animals  the  pul- 
sation of  the  heart  takes  place  constantly  in  a  given 
direction,  usually  from  back  to  front,  in  the  Ascidians  it 
alternates  between  opposite  directions.  First,  the  heart 
contracts  in  the  direction  from  back  to  front,  then,  after 
standing  still  a  minute,  it  begins  to  pulsate  in  the  opposite 
direction,  driving  the  blood  from  front  to  back ;  thus  the 
two  great  vessels  proceeding  from  the  opposite  ends  of 
the  heart  act  alternately  as  arteries  and  veins.  This  is  a 
peculiarity  which  appears  only  in  the  Mantle-Animate 
(Tunicata). 


434 


THE    EVOLUTION    OF   MAN. 


Of  the  other  important  organs,  we  have  yet  to  mention 
those  of  reproduction,  which  lie  at  the  posterior  extremity 
of  the  body-cavity.  All  the  Sea-squirts  are  hermaphrodites. 
Each  individual  has  a,  male  and  a  female  gland,  and  is  thus 
capable  of  self-fertilization.  The  mature  eggs  (Fig.  154,  o') 
fall  directly  from  the  ovary  (o)  into  the  gill-cavity.  The 
male  sperm,  on  the  contrary,  is  carried  from  the  testes  (t) 
into  the  same  cavity  by  a  special  seed-duct  (vd).  Here 
impregnation  takes  place,  and  here  in  many  Sea-squirts 
developed  embryos  are  found  (Plate  XI.  Fig.  14,  z).  These, 
with  the  water  that  has  been  inhaled,  are  then  thrown  out 
at  the  gill-pore  (q) ;  they  are  thus  "born  alive." 

Many    Sea-squirts,    especially    of   the    smaller    species, 

FIG.  154. — Structure  of  an  Ascidian  (observed  from 
the  left  side,  as  in  Fig.  153,  and  Fig.  14,  Table  XL) : 
sb,  gill-sac  ;  v,  stomach ;  i,  large  intestine ;  c,  heart ; 
t,  testes  ;  vd,  seed-duct ;  o,  ovary  ;  o',  matured  eggs  in 
the  gill-cavity.  The  two  little  arrows  indicate  the  en- 
trance and  exit  of  the  water  through  the  two  openings 
of  the  tunic.  (After  Milne  Edwards.) 

multiply,  not  by  sexual  reproduction,  but 
asexually  by  the  formation  of  buds.  Great 
numbers  of  the  individuals  thus  produced 
from  buds  remain  permanently  attached  to 
each  other,  thus  forming  large  masses,  or 
comes  like  the  well-known  coral  societies. 
Among  these  social  or  compound  Ascidians, 
those  species  are  peculiarly  interesting  in 
which  the  mass  seems  to  be  beautifully 
combined  of  many  star-shaped  groups.  Each 
star-shaped  group  consists  of  a  larger  or 
smaller  number  of  individuals,  of  which  every  one  possesses 


REPRODUCTION  IN  ASCIDIANS.  435 

its  independent  organization  and  its  own  mouth-opening.  All 
the  individuals  together  have,  however,  but  a  single  common 
gill-pore,  which  is  situated  at  the  central  point  of  the  star- 
shaped  group.  These  star-shaped  compound  ascidian  groups 
(Botryllus,  Polydinum,  etc.)  throw  much  light  on  the 
Phylogeny  of  one  of  the  most  remarkable  races  of  animals, 
the  Star-animals  (Echinoderma).  The  parent-forms  of 
these  are  the  Star-fish,  or  Asterids,  which  are,  like  the 
compound  Ascidians,  star-shaped  societies  formed  of  Worms 
connected  by  a  common  central  intestinal  opening."8 

If  we  now  once  more  glance  back  at  the  entire  organiza- 
tion of  the  simple  Ascidians,  Sea-squirts  (Phallusia,  Cyn- 
thia, etc.),  and  compare  it  with  that  of  the  Amphioxus,  we 
find  that  the  two  present  few  points  of  resemblance.  The 
developed  Ascidian  is  indeed  like  the  Amphioxus  in  some 
important  points  of  internal  structure,  especially  in  the 
peculiar  construction  of  the  gill-sac  and  intestine.  But 
it  seems  so  far  removed  in  most  other  particulars  of  its 
organization,  and  is  so  dissimilar  in  outward  appearance, 
that  the  very  near  relationship  of  the  two  organisms  is  only 
revealed  by  study  of  their  germ-histories.  We  will  now 
consider  and  compare  the  individual  development  of  the 
two  animals,  and  shall  in  this  way  find,  to  our  great  sur- 
prise, that  the  same  embryonic  animal  form  develops  from 
the  egg  of  the  Ainphioxus  as  from  the  egg  of  the  Ascidian, 


EXPLANATION  OF  PLATES  X.  AND  XL 

PLATB    X. — GERM-HISTORY    OF    THE    ASCIDIAN   AND  OF    THE  AMPHIOXUS, 
(PRINCIPALLY  ACCORDING  TO  KOWALEVSK.Y.) 

Fro.  1-6. — Germ-history  of  an  Ascidian. 

FIG.  1. — A  parent-cell  (cytula)  of  an  Ascidian.  In  the  bright-coloured 
protoplasm  of  the  parent-cell  lies  eccentrically  a  bright  spherical  kernel 
(nucleus),  and  in  the  latter  a  darker  nucleolus. 

FIG.  2. — An  Ascidian  egg  in  the  process  of  cleavage.  The  parent-coll 
has  divided  by  repeated  bisection  into  four  similar  cells. 

FIG.  3. — Membraneous  germ-vesicle  of  an  Ascidian  (Blastula).  The  celly 
resulting  from  the  cleavage  of  the  egg  form  a  spherical  bladder  filled  with 
fluid,  the  wall  of  which  consists  of  a  single  layer  of  cells.  (Of.  Fig.  22,  F,  G.) 

FIG.  4.— Gastrula  of  the  Ascidian  resulting  from  the  blastula  (Fig.  3) 
by  inversion  (invagination).  The  wall  of  the  primitive  intestine  (d),  which 
opens  at  o  by  the  primitive  month,  consists  of  two  layers  of  cells ;  the  inner 
intestinal  layer  formed  of  larger  cells,  and  the  onter  skin-layer,  of  smaller. 

FIG.  5. — Larva  of  the  Ascidian  swimming  freely.  Between  the  medullary 
tube  (m)  and  the  intestinal  tube  (d)  the  notochord  is  inserted  (cti),  which 
passes  throughout  the  long  rudder-like  tail  to  its  point. 

FIG.  6. — Transverse  section  through  a  larval  Ascidian  (Fig.  5),  through 
the  posterior  part  of  the  trunk  just  in  front  of  the  beginning  of  the  tail. 
The  section  is  just  the  same  as  that  of  the  Amphioxus  larva  (Fig.  11,  12). 
Between  the  medullary  tube  (m)  and  the  intestinal  tube  (d)  lies  the  noto- 
chord (ch)  ;  on  both  sides  are  the  lateral  muscles  of  the  trunk  (r). 

FIG.  7-13. — Germ-history  of  the  Amphioxns. 

FIG.  7.— Parent-cell  (cytula)  of  the  Amphioxus.     (Cf.  Fig.  1.) 

FIG.  8. — An  amphioxus-egg  in  the  process  of  cleavage.     (Cf.  Fig.  2). 

FIG.  9.— Blastnla  of  the  Amphioxus.     (Cf.  Fig.  3.) 

FIG.  10.— Gastrula  of  the  Amphioxus.     (Cf.  Fig.  4.) 

FIG.  11. — Young  larva  of  the  Amphioxus.  The  notochord  (cJi)  lies 
between  the  medullary  tube  Cm)  and  the  intestinal  tube  (d).  The  medullary 
tube  has  an  opening  at  the  anterior  extremity  of  the  body  (ma). 


EXPLANATION  OF  PLATES  X.  AND  XL        437 

FIG.  12. — An  older  larva  of  the  Ampbioxus.  On  both  sides  of  the 
medullary  tube  (m)  and  of  the  notochord  (ch)  a  longitudinal  row  of  muscle- 
plates  (rap)  is  visible ;  these  mark  the  embryonic  vertebras,  or  metaraera. 
An  organ  of  sense  has  developed  in  front  (ss).  The  wall  of  the  intestine 
(d)  is  much  thicker  below  on  the  ventral  side  (du)  than  above  on  the  dorsal 
side  (do).  The  anterior  part  of  the  intestinal  canal  widens  into  the  gill« 
body. 

Fio.  13. — Transverse  section  through  a  developed  Amphioxus  (Fig.  15) 
a  little  behind  the  centre  of  the  body.  Above  the  intestinal  tube  (d)  is  the 
dorsal  blood-vessel,  or  main  artery  (£),  and  below  it  the  ventral  blood-vessel, 
or  the  intestinal  vein  (u).  At  the  inner  wall  of  the  gill-cavity  (c)  lie  the 
ovaries  (e),  and  outside  these  the  side  canals  (u).  The  dorsal  muscles  (r) 
are  divided  into  several  parts  by  inter-muscular  ligaments  (ml)  j  /,  dorsal 
fin. 


PLATE  XI. — STEUCTURB  or  THE  ASCIDIAN,  OF  THE  AMPHIOXUS,  AND  o»  THE 
LARVA  OF  THE  PETEOMYZON. 

For  the  sake  of  comparison,  all  the  three  animals  are  placed  in  the  same 
position  and  are  represented  of  the  same  size.  The  view  is  from  the  left 
side.  The  head  extremity  is  turned  upward,  the  tail  downward  ;  the  dorsal 
side  to  the  right,  the  ventral  side  to  the  left.  The  enveloping  membrane  is 
removed  from  the  left  side  of  the  body,  to  show  the  inner  organization  with 
the  organs  in  their  natural  position. 

FIG.  14. — A  simple  Ascidian  (Monascidia) ,  magnified  six  times. 

FIG.  15. — A  developed  Amphioxus  (magnified  four  times). 

For  the  sake  of  giving  a  more  distinct  view,  the  Amphioxus  in  Fig.  15  is 
drawn  about  twice  its  actual  breadth.  In  reality,  its  breadth  amounts  to 
but  half  of  the  length  as  represented  here. 

FIG.  16. — Young  larva  of  a  lamprey  (Petromyzon  Planeri),  eleven  daya 
after  emerging  from  the  egg,  magnified  45  times.  (After  Max  Schultze.) 
The  larva  of  the  lamprey,  which  undergoes  a  peculiar  transformation  at  a 
later  period,  was  formerly  considered  as  a  distinct  species  under  the  name  of 
Ammoccetes. 

The  meaning  of  the  letters  is  the  same  in  all  the  figures. 


ALPHABETICAL  EXPLANATION 
Of  the  Meaning  of  the  Letters  in  Plates  X.  and  XL 


a,     auas 

TO2,  spinal  marrow 

ai*}  oye 

ma,  anterior  opening  of  tho  modul 

bt     ventral  muscles 

lary  tube 

c,     gill-cavity 

mb,  muscular  ligaments 

ch,   notochord  (spinal  axis) 

mg,  stomach 

cl,    cloacal  cavity 

mil,  month-  cavity 

cs,    notochord  sheath. 

mp,  muscle-plate 

d,     intestinal  tube 

mt,  mantle 

do,   dorsal  wall  of  the  intestine 

n,     nose  (nose-pit) 

du,  ventral     „         „        „ 

o,     month-opening 

e,     ovary 

p,    ventral  pore 

en,  endostyle 

q,     cloacal  opening 

/,      fin-seam 

r,     dorsal  muscles 

g,     ear-vesicle 

*,     tail-fins 

h,     horn-plato 

si,    seed-duct 

hdy  testes 

am,  opening  of  the  seed-duct 

hk,  venticlo 

ss,   organ  of  sense 

hv,  auricle 

t,     aorta  (dorsal  blood-  vessel) 

he,  heart 

th,   thyroid  gland 

»,     egga 

u,     side  canals 

fe,     gills 

v,    intestinal    vein    (ventral    blood. 

ka,  gill-artery 

vessel) 

I,      leather-plate 

w,    root-fibres  of  the  ascidian. 

Ib,   liver 

0,     boundary  between  the  gill-intes- 

W, anterior  end  of  the  liver 

tine  and  the  stomach.intestinc 

Iv,   liver  vein 

y,     hypo-branchial  groove 

m,   medullary  tubo 

c,     embryos  of  the  ascidian 

mi,  brain-bladder 

ONTOGENY  OF  THE  ASCIDIAN  (1-6)  AND  OF  THE  AMPHIOXUS  (7-13). 


HAECKEL'S    EVOLUTION   OF    M/ 


PLATE  XI. 


ANATOMY  OF  THE  ASCIDIAN  (14)  AND  AMPHIOXUS  (15). 


CHAPTER  XIV. 

GERM-HISTORY  OP  THE  AMPHIOXUS  AND  OF  THE 

ASCIDIAN. 

Relationship  of  the  Vertebrates  and  Invertebrates.— Fertilization  of  the 
Amphioxus.— The  Egg  undergoes  Total  Cleavage,  and  changes  into  a 
Spherical  Germ-membrane  Vesicle  (Blastula) .—  From  this  the  Intes- 
tinal Larva,  or  Gastrula,  originates  by  Inversion. — The  Gastrula  of  tho 
Amphioxus  forms  a  Medullary  Tube  from  a  Dorsal  Furrow,  and 
between  this  and  the  Intestinal  Tube,  a  Notochord  :  on  both  Sides  the 
latter  is  a  Series  of  Muscle-plates;  the  Metamera. — Fate  of  the 
Four  Secondary  Germ-layers. — The  Intestinal  Canal  divides  into  an 
Anterior  Gill-intestine,  and  a  Posterior  Stomach-intestine. — Blood- 
vessels and  an  Intestinal-muscle  Wall  originate  from  the  Intestinal- 
fibrous  Layer. — A  Pair  of  Skin-folds  (Gill-roofs)  grow  out  from  the 
Side-wall  of  the  Body,  and,  by  Coalescence,  form  the  Ventral  Side  of 
the  Large  Gill-cavity. — The  Ontogeny  of  the  Ascidian  is,  at  first,  iden- 
tical with  that  of  the  Amphioxns. — The  same  Gastrnla  is  Developed, 
which  forms  a  Notochord  between  the  Medullary  and  Intestinal  Tubes. 
— Eetrogressive  Development  of  the  same. — The  Tail  with  the  Xotochord 
is  shed. — The  Ascidian  attaches  itself  firmly,  and  envelopes  itself  in 
its  Cellulose  Tunic. — Appendicnlaria,  a  Tunicate  which  remains  through- 
out Life  in  the  Stage  of  the  Larval  Ascidian  and  retains  the  Tail-fin 
with  the  Chorda  (Chordonia) . — General  Comparison  and  Significance  of 
the  Amphioxus  and  the  Ascidian. 

"  In  the  formation  of  its  most  important  organs,  the  Amphioxus  remains 
throughout  life  at  that  lowest  stage  of  development,  which  all  other  Verte- 
brates pass  rapidly  through  during  the  earliest  period  of  their  embryonic 
existence.  We  must  therefore  regard  the  Amphioxus  with  peculiar  reverence 
an  that  animal,  which  among  all  existing  creatures  is  the  one  alone  capable 


44O  THE   EVOLUTION   OF   MAN. 

of  giving  us  an  approximate  idea  of  our  oldest  Silurian  vertebrate  ancestors. 
But  the  latter  are  descended  from  Worms,  the  nearest  blood-relatives  of 
which  an>  the  Ascidians  of  the  present  day." — The  Pedigree  of  the  Human 
Race  (1868). 

THE  peculiarities  in  the  structure  of  the  body,  which  dis- 
tinguish Vertebrates  from  Invertebrates,  are  so  striking, 
that  the  relationship  of  these  two  main  groups  of  the  animal 
kingdom  formerly  threw  great  difficulties  in  the  way  of 
systematic  classification.  When,  in  accordance  with  the 
Theory  of  Descent,  the  relationship  of  the  various  groups 
of  animals  began  to  be  regarded  as  not  figurative,  but  as 
really  genealogical,  this  question  came  to  the  front,  and 
seemed  to  offer  one  of  the  greatest  obstacles  to  the  success 
of  the  theory.  Even  at  an  earlier  period,  when  without  this 
fundamental  thought  of  the  true  genealogical  connection 
of  the  relationships  between  the  great  main  groups  of  the 
animal  kingdom,  the  so-called  "  types  "  of  Baer  and  Cuvier 
were  studied,  investigators  believed  they  had  found,  here 
and  there  among  Invertebrates,  points  connecting  these 
with  Vertebrates ;  some  single  species  of  Worms,  in  par- 
ticular, appeared  to  approximate  in  the  structure  of  their 
bodies  to  the  Vertebrates ;  as,  for  example,  the  oceanic  Arrow- 
worm  (Sagitta).  But  the  attempted  analogy  was  shown, 
by  closer  investigation,  to  be  untenable.  After  Darwin 
gave  an  impulse  to  a  true  tribal  history  of  the  animal 
kingdom,  by  his  reform  of  the  Theory  of  Descent,  this  very 
relation  seemed  to  offer  one  of  the  greatest  difficulties. 
When,  in  1866,  I  attempted,  in  my  Generelle  Morphologie, 
to  carry  out  the  Theory  of  Descent  in  detail,  and  to  apply 
it  to  the  natural  system,  no  part  of  my  task  demanded 
more  care  than  the  connection  of  the  Vertebrates  with  tho 
Invertebrates. 


RELATION   OF  THE   LANCELET  TO   THE  ASCIDIAN.       44! 

But  just  at  this  time  the  true  connection  was  discovered 
in  an  entirely  unhoped-for  and  most  unexpected  quarter. 
Toward  the  end  of  the  year  1866,  among  the  treatises 
of  the  St.  Petersburg  academy,  two  works  appeared  by 
the  Russian  zoologist,  Kowalevsky,  who  had  spent  a  long 
time  at  Naples,  and  had  occupied  himself  in  studying  the 
individual  evolution  of  some  of  the  lower  animals.  A  fortu- 
nate accident  had  led  Kowalevsky  to  study  almost  simul- 
taneously the  individual  evolution  of  the  lowest  Vertebrate, 
the  Amphioxus,  and  that  of  an  Invertebrate,  the  direct 
relationship  of  which  to  the  Amphioxus  had  not  been  even 
guessed,  namely,  the  Ascidian.  Greatly  to  the  surprise  of 
Darwin  himself,  and  of  all  zoologists  interested  in  that 
important  subject,  there  appeared,  from  the  very  commence- 
ment of  their  individual  development,  the  greatest  identity 
in  the  structure  of  the  bodies  of  those  two  wholly  different 
animals, — between  the  lowest  Vertebrate,  the  Amphioxus, 
on  the  one  hand,  and  that  misshapen  lump  adhering  to 
the  bottom  of  the  sea,  the  Sea-squirt,  or  Ascidian,  on  the 
other  hand.  In  this  undeniable  ontogenetic  agreement,  the 
existence  of  which,  in  an  astonishing  degree,  can  be  proved, 
the  long-sought  genealogical  link  was,  of  course,  directly 
found,  according  to  the  fundamental  law  of  Biogeny,  and 
that  group  of  Invertebrates,  which  is  most  nearly  allied  to 
the  Vertebrates,  was  clearly  recognized.  There  can  be  no 
longer  any  doubt,  especially  since  Kupffer  and  several  other 
zoologists  have  confirmed  and  continued  these  investiga- 
tions, that  of  all  classes  of  Invertebrates,  the  Mantle-animals 
(Tunicata),  and  of  the  latter,  the  Ascidians,  are  most  nearly 
allied  to  the  Vertebrates.  We  cannot  say  the  Vertebrates 
arc  descended  from  the  Ascidians ;  but  we  may  safely  assert, 


442  THE   EVOLUTION   OF   MAN. 

that  of  all  Invertebrates,  the  Man  tie -animals,  and  among 
the  latter  the  Ascidians,  are  the  nearest  blood-relations  to 
the  primeval  parent-form  of  Vertebrates.  An  extinct  species 
of  the  very  varied  Worm  tribe  must  be  assumed  as  the 
common  parent-form  of  both  groups. 

In  order  fully  to  appreciate  this  extraordinarily  im- 
portant circumstance,  and  especially  in  order  to  gain  a  secure 
baxsis  for  the  desired  genealogical  tree  of  Vertebrates,  it  is  in- 
dispensable to  note  minutely  the  germ-history  of  these  two 
remarkable  animals,  and  to  compare  the  individual  develop- 
ment of  the  Amphioxus  stage  by  stage  with  that  of  the 
Ascidian.  (Of.  Plate  X.,  and  p.  436.)  We  will  begin  with 
the  Ontogeny  of  the  Amphioxus  (Plate  X.  Figs.  7-12). 
Kowalevsky  had  already  spent  several  months  in  Naples 
with  the  express  intention  of  studying  the  wholly  unknown 
germ-history  of  the  Amphioxus,  before  he  succeeded  in 
observing  the  mature  eggs  in  the  first  stages  of  development. 
He  says  that  the  Lancelot  begins  to  deposit  its  sexual  products 
in  the  month  of  May,  in  the  warm  evening  hours,  between 
seven  and  eight  o'clock.119  He  noticed  that  at  this  time, 
the  male  animal  first  ejected  a  whitish  fluid,  the  sperm,  and 
that,  somewhat  later,  the  female,  attracted  by  the  sperm, 
also  deposited  its  eggs  in  the  water. 

According  to  other  observers  the  deposit  of  the  sexual 
products  is  said  to  take  place  through  the  gill-pore  (porus 
branchialis).  The  eggs  are  simple  roundish  cells.  They 
have  a  diameter  of  only  fa  of  a  millimetre,  are,  therefore, 
only  half  as  large  as  mammalian  eggs,  and  offer  no  special 
peculiarities  (Plate  X.  Fig.  7).  The  active  elementary 
bodies  of  the  male  seed,  the  pin-shaped  "  seed-animals,"  or 
Bperm-cells,  all  resemble  those  of  most  other  animals.  (Cf. 


GERM-HISTORY    OF   THE   LANCELET.  443 

Fig.  17.  p.  173.)  Fertilization  is  accomplished  in  this  way : 
the  moving  whip-cells  of  the  sperm  approach  the  egg,  and 
with  their  head-portion,  that  is,  the  thickened  portion  of 
the  cell  which  encloses  the  nucleus,  they  force  their  way 
into  the  yelk-mass  or  cell-substance  of  the  egg. 

Either  before  or  immediately  after  fertilization,  the  egg- 
cell  loses  its  original  kernel,  and  appears  for  a  time  in  the 
form  of  a  kernel-less  cytod,  as  a  monerula.  (Of.  Fig.  19,  p.  179.) 
A  new  kernel  soon,  however,  originates  in  the  impregnated 
yelk;  this  is  the  parent-kernel,  and  the  monerula  thus  changes 
into  the  parent-cell  (cytula,  Fig.  21,  p.  181.)  This  now 
undergoes  a  regular  and  total  cleavage,  the  details  of  which 
in  a  coral  (Monoxenia)  we  have  described  in  detail  (cf.  Fig. 
22).  The  repeated  bisection  of  the  parent-cell  into  2,  4,  8, 
16,  32,  64  cells  and  so  on,  gives  rise  to  the  globular,  black- 
berry or  mulberry-shaped  body  which  we  called  the  "  mul- 
berry-germ" (morula,  .Fig.  22,  E).  Fluid  collects  in  the 
interior  of  this  globular  mass,  composed  entirely  of  one  sort 
of  cleavage-cells,  and  the  result  is  the  formation  of  a  spheri- 
cal vesicle,  the  wall  of  which  is  composed  of  a  single  layer 
of  cells  (Plate  X.  Fig.  9).  We  called  this  vesicle  the  mem- 
branous germ-vesicle  (blastula).  Its  contents  form  a  clear 
fluid ;  the  wall,  which  consists  of  a  single  layer  of  cells,  is 
the  germ-membrane,  or  blastoderma  (Fig.  22,  F,  ff). 

These  processes  take  place  so  rapidly  in  the  Amphioxus, 
that  in  from  four  to  five  hours  after  impregnation,  that  is, 
about  midnight,  the  spherical  blastula  is  complete.  On  one 
side  of  the  latter  appears  a  groove-like  depression,  by  which 
the  vesicle  is  turned  into  itself  (Fig.  22,  H,  p.  190).  This 
furrow  grows  constantly  deeper,  while  the  spherical  form  of 
the  vesicle  changes  into  an  oval  or  ellipsoid  shape  (Fig.  155). 
31 


444 


THE    EVOLUTION    OF   MAN. 


At  last,  the  inversion  is  complete,  so  that  the  inner  part  of 
the  wall,  that  which  has  been  inverted,  lies  on  the  inside  of 
the  outer,  the  uninverted  part.  In  this  way  an  almost 
hemispherical  hollow  body  is  formed,  the  thin  wall  of  which 
is  composed  of  two  layers  of  cells.  The  hemispherical  form 
soon  again  changes  into  an  almost  spherical  or  oval  shape, 
in  consequence  of  the  inner  cavity  becoming  considerably 
enlarged,  while  its  opening  becomes  narrower  (Plate  X. 
Fig.  10).  The  form  which  the  embryo  of  the  Amphioxus 
has  now  attained  in  this  way  is  a  true  Gastrula  or  intes- 
tinal larva ;  is  indeed  a  gastrula  of  that  original  and 
simplest  form  which  we  have  already  distinguished  as  the 
Bell-gastrula  or  Archigastrula  (p.  191,  Fig.  22, 1,  K}. 

FIG.  155. — Gastrula  of  Amphi. 
oxns,   in   longitudinal   section :    d, 
primitive     intestine ;    o,    primitive 
mouth  ;  i,  intestinal  layer,  or  ento" 
*        derm ;  e,  skin-layer,  or  exoderm. 

As  in  all  those  lowly 
organized  animals  which 
form  a  primitive  Bell-gas- 
trula of  this  sort,  the  body 
of  the  Amphioxus,  which 
has  but  one  axis,  is  merely 
a  simple  intestinal  pouch 

the  inner  cavity  of  this  is  the  primitive  intestine  (proto- 
gaster)  (Fig.  155,  d,  Fig.  156,  g) ;  its  simple  opening  is  the 
primitive  mouth  (protostoma;,  o).  The  wall  is  at  once 
the  intestinal  wall  and  the  body-wall.  It  is  composed  of 
two  cell- strata,  of  the  two  well-known  primary  germ-layers. 
The  inner  stratum,  or  the  inverted  portion  of  the  Uastula, 


BELL-GASTRULA   OF   THE   LANCELET.  445 

which  immediately  surrounds  the  intestinal  cavity,  is  the 
entoderm,  the  inner  or  vegetative  germ-layer,  from  which 
are  developed  the  wall  of  the  intestinal  canal  and  all  its 
appendages  (Fig.  155,  156,  i).  The  outer  cell-stratum,  the 
part  of  the  blastula  not  inverted,  is  the  exoderm,  the  outer 
or  animal  germ-layer,  which  furnishes  the  rudiment  of  the 
body-wall,  the  skin,  the  flesh,  the  central  nervous  system, 
etc.  (e}.  The  cells  of  the  inner  stratum,  or  entoderm,  are 
considerably  larger,  duller,  darker,  and  more  adipose  than 
those  of  the  outer  stratum,  or  exoderm,  which  are  clearer', 
brighter,  and  less  rich  in  fatty  particles.  Thus,  even  during 
the  process  of  inversion,  a  differentiation  takes  place  between 
the  inner  inverted  stratum  and  the  outer  uninverted.  The 
cells  of  the  outer  layer  are  soon  covered  with  fine  bright 
hairs;  fine,  short,  thread-like  appendages,  grow  from  the 
protoplasm,  which  keep  up  a  constant  vibratory  motion. 


Fig.  156.— Gastrula  of  a  Chalk-sponge  (Olynthus)  :  A,  from  the  outside; 
B,  in  longitudinal  section  through  the  axis  ;  g,  primitive  intestine  ;  o,  primi- 
tive month  •  i,  intestinal-layer,  or  entoderm ;  e,  skin-layer,  or  exoderm. 


446  THE    EVOLUTION    OF   MAN. 

By  the  motions  of  these  delicate  vibratory  hairs,  the  gastrula 
of  the  Amphioxus,  like  that  of  many  other  animals  of  low 
organization,  after  it  has  broken  through  the  egg-coveringy 
rotates  and  swims  in  the  ocean  (Fig.  156). 

In  the  course  of  further  development  the  roundish  Bell- 
gastrula  of  the  Amphioxus  lengthens,  and  at  the  same  time 
it  becomes  rather  natter  on  one  side  parallel  to  the  longi- 
tudinal axis.  The  flattened  side  is  afterwards  the  dorsal- 
side  ;  the  opposite  ventral  side  remains  roundly  arched.  In 
the  middle  of  the  dorsal  surface  appears  a  shallow  longi- 
tudinal furrow  or  channel  (Fig.  157),  and  on  each  side  of 
this  channel  the  surface  of  the  body  rises  in  the  shape  of 
two  parallel  ridges  or  longitudinal  swellings.  I  need 
hardly  say,  that  this  channel  is  the  primitive  groove,  or 
dorsal  furrow,  and  that  these  swellings  are  the  dorsal 
swellings  or  spinal  swellings  which  form  the  first  rudiments 
of  the  central  nervous  system,  the  medullary  tube.  These 
two  swellings  grow  higher  and  higher ;  the  groove  becomes 
deeper  and  deeper.  The  edges  of  the  two  parallel  swellings 
incline  towards  each  other,  and  finally  coalesce,  and  thus 
the  medullary  tube  is  completed  (Plate  X.  Fig.  11,  m).  The 
formation  of  the  medullary  tube  from  the  outer  skin  takes 
place,  therefore,  on  the  naked  dorsal  surface  of  the  independent 
Amphioxus  larva  in  exactly  the  same  way  as  in  the  embryo 
of  Man  and  of  other  Vertebrates  within  the  egg-envelopes. 
In  both  cases,  also,  the  nerve-tube  finally  separates  entirely 
from  the  horny  plate.  The  fact  is  peculiar,  that  at  that 
end  of  the  body  which  afterwards  is  to  be  the  anterior  or 
mouth  end  of  the  Amphioxus,  the  medullary  tube  remains 
open  at  first,  and  has  an  external  opening  (Fig.  11,  ma). 

Even  at  the  time  when  the  first  trace  of  the  dorsal  furrow 


GERM  DEVELOPMENT  IN  THE  LANCELET.       447 

appears,  the  two  primary  germ-layers  of  the  Amphioxus 
larva  split  up  into  the  four  secondary  germ-layers  (Fig.  157, 
transverse  section).  Round  the  inner  vegetative  layer  of 
the  intestinal  tube  there  arises,  in  consequence  of  a  fission 
of  the  cells  of  the  latter,  a  second  external  cell-stratum,  the 

FIG.  157. — Transverse  section  through 
a  larval  Amphioxns  '(after  Kowalevsky)  : 
hs,  skin-sensory  layer;  hm,  skin-fibrons 
layer;  c,  coelom-fissnre  (rudimentary 
body-cavity) ;  df,  intestinal-fibrous  layer; 
dd,  intestinal  glandular  layer;  a,  primi- 
tive intestine  (primitive  intestinal  cavity). 
Above,  the  dorsal  furrow  is  seen  between 
the  two  dorsal  swellings. 

intestinal-fibrous  layer  (df) ;  from  this  originate  the 
muscles  and  the  fibrous  membranes  of  the  intestinal  tube, 
and  the  blood-vessels.  The  original  inner  cell-stratum 
must  now  be  called  the  intestinal-glandular  layer  (dd). 
Analogously,  the  outer  animal  germ-layer  falls,  in  con- 
sequence of  a  fission  in  its  cells,  into  two  strata,  an  outer 
skin-sensory  layer  (hs}  and  an  inner  skin-fibrous  layer  (hm). 
The  former  gives  rise  to  the  outer  skin  (epidermis)  and  the 
medullary  tube ;  the  latter  to  the  leather-skin  (corium) 
and  the  trunk-muscles.  A  space  forms  between  the  skin- 
fibrous  layer  and  the  intestinal-fibrous  layer,  in  which  a 
colourless  liquid  collects,  thus  forming  the  body-cavity 
(cceloma,  c).  It  is  a  fact  of  great  moment  for  the  germ- 
layer  theory  that,  here  in  the  Amphioxus,  the  origin  of  the 
skin-fibrous  layer  from  the  animal,  and  that  of  the  intestinal- 
fibrous  layer  from  the  vegetative  germ-layer  is  plainly 
demonstrable. 

As  soon  as  the  four  secondary  germ-layers  have  formed 


443  THE   EVOLUTION   OF   MAN. 

a  cylindrical  cord,  pointed  at  both  ends,  and  composed  of 
large,  light-coloured  vesicular  cells,  appears  in  the  middle 
line  of  the  skin-fibrous  layer,  directly  over  the  intestinal 
tube  (d)  and  below  the  nerve-tube  (ra),  (and  therefore 
along  the  long  axis  of  the  body).  This  is  the  chorda 
dorsalis,  or  notochord  (Plate  X.  Fig.  11, 12,  di).  The  lateral 
portions  of  the  skin-fibrous  layer,  which  lie  on  both  sides 
of  the  notochord,  and  which  we  may  in  this  case  also  call 
"side-layers,"  or  "side-plates,"  split  into  two  strata,  a  thin 
leather-skin  (corium)  and  an  underlying  muscle-plate. 
The  latter  soon  breaks  up  into  a  number  of  homogeneous 
sections,  lying  one  behind  another.  These  are  the  side 
muscles  of  the  trunk,  which  indicate  the  first  articulation 
or  metameric  structure  of  the  body  (Fig.  12,  mp). 

By  these  separations  the  gastrula  of  the  Amphioxus  has 
changed  into  a  vertebrate  body  of  the  simplest  form,  with 
the  characteristic  disposition  of  the  fundamental  organs 
which  belongs  exclusively  to  Vertebrates.  Directly  below  the 
skin  we  find,  at  the  dorsal  side  of  the  medullary  tube,  on  ths 
ventral  side  of  the  intestinal  tube,  and  between  the  two 
tubes,  the  firm  axis  of  the  body,  the  notochord ;  and,  on 
either  side  of  this,  the  regular  series  of  muscle-plates.  If 
we  now  look  at  the  larva  of  the  Amphioxus  from  one  side 
(Plate  X.  Fig.  11,  12),  we  see  that  on  the  top  lies  the 
medullary  tube,  still  open  anteriorly  (ma) ;  directly  undei 
this  lies  the  strong  notochord  (ch\  and  under  this  the 
much  broader  intestinal  tube  (d).  The  latter  also  has  an 
opening  at  one  end,  the  original  mouth  of  the  gastrula  (o). 
It  is,  however,  a  very  singular  and  important  fact  that  this 
primitive  mouth  does  not  afterwards  become  the  permanent 
mouth-opening  of  the  Amphioxus.  On  the  contrary,  it  soon 


VERTEBRATE  NATURE  OF  THE  LANCELET.      449 

closes.  The  future  permanent  mouth  is  formed  only  second- 
arily, from  the  outside,  and  at  the  opposite  end  of  the  body 
(near  ss,  Fig.  12).  At  this  point,  a  groove-like  depression 
originates  in  the  outer  skin  (epidermis),  and  this  grows 
inwards  and  breaks  a  way  through  into  the  closed  intestine. 
Similarly,  the  anal  opening  forms  behind  (in  the  neighbour- 
hood of  the  closed  gastrula-mouth).  We  saw  that  in  Man 
and  in  all  higher  Vertebrates  mouth  and  anus  originate 
as  shallow  grooves  in  the  outer  skin ;  and  that  these  also 
break  through  inwards,  thus  gradually  communicating  with 
both  blind  ends  of  the  intestinal  tube.  (Of.  p.  338.) 

Between  the  intestinal  and  the  nerve  tubes  we  find  the 
notochord  as  a  cartilaginous  cylindrical  rod,  traversing 
the  entire  length  of  the  larval  body.  On  each  side  of  the 
notochord  lie  the  muscle-plates,  already  broken  up  into 
a  number  of  separate  pieces,  or  primitive  vertebral  seg-> 
ments  (10  to  20  on  each  side);  these  are  separated  from 
each  other  by  simple  oblique,  parallel  lines  of  demar- 
cation. In  the  fully-formed  animal  each  of  these  divid- 
ing lines  describes  an  acute  angle  forwards  (Plate  XL  Fig. 
15,  r).  The  number  of  separate  muscle-plates  indicates 
the  number  of  metamera  of  which  the  body  consists.  At 
first  this  number  is  small,  but  it  afterwards  increases 
considerably  in  the  direction  from  front  to  rear.  This 
is  owing  to  that  same  terminal  budding  in  virtue  of 
which  the  chain  of  primitive  vertebral  segments  grows 
in  the  human  embryo.  Here,  too,  the  foremost  metamera 
are  the  oldest,  and  the  terminal  ones  the  most  recent.  To 
each  metameron  corresponds  a  definite  segment  of  the 
medullary  tube  and  a  pair  of  spinal  nerves,  which  pass  from 
it  out  to  the  muscles  and  to  the  skin.  Of  all  the  organic 


45O  THE   EVOLUTION   OF   MAN. 

systems  of  the  body,  it  is  in  the  muscle-system  that  arti- 
culation first  appears.120 

While  these  characteristic  differentiations  are  taking 
place  in  the  two  lamellae  of  the  animal  germ-la}rer — v/hile 
the  medullary  tube  and  the  outer  skin  (epidermis)  arc 
separating  from  the  skin-sensory  layer,  and  the  notochord 
and  the  muscle-plates  from  the  skin-fibrous  layer,  equally 
important  processes,  characteristic  of  the  vertebrate  type, 
are  taking  place  in  the  vegetative  germ-layer.  The  inner 
lamella  of  this — the  intestinal-glandular  layer — undergoes 
but  few  modifications;  it  produces  only  the  internal  cell- 
coating,  or  epithelium  of  the  intestinal  tube  (d).  But  the 
outer  lamella,  the  intestinal-fibrous  layer,  produces  both 
the  muscular  covering  of  the  intestine  and  the  blood- 
vessels. Probably  simultaneously,  two  main  vessels  ori- 
ginate from  this  layer :  an  upper,  or  dorsal  vessel,  corre- 
sponding to  the  aorta,  situate  between  the  intestine  and  the 
chorda  dorsalis  (Figs.  13,  t,  15,  f) ;  and  a  lower,  or  ventral 
vessel,  answering  to  the  heart  and  the  intestinal  vein,  on 
the  lower  edge  of  the  intestine,  and  between  it  and  the 
ventral  skin  (Figs.  13,  v,  15,  v).  Moreover,  at  this  time 
the  gills,  or  respiratory  organs,  also  develop  in  the  anterior 
portion  of  the  intestinal  canal  The  whole  anterior  or 
respiratory  section  of  the  intestine  changes  into  a  gill-body, 
which  is  pierced  by  numerous  openings,  so  that  it  resembles 
a  lattice- work,  as  in  Ascidia.  The  cause  of  this  is  that  the 
foremost  portion  of  the  intestinal  wall  adheres  in  places 
to  the  external  skin,  and  that,  at  these  points  of  adhesion, 
openings  form  in  the  wall  and  extend  from  outside  into 
the  intestine.  At  first  these  gill-openings  are  but  very  few, 
but  soon  they  are  numerous,  appearing  first  in  one  row. 


RESPIRATION    IN   THE   LANCELET.  431 

then  in  two  rows,  one  behind  the  other.  The  foremost 
gill-opening  is  the  oldest.  Finally,  a  lattice-work  of  fine 
gill-openings  appears  on  each  side. 

We  must  call  special  attention  to  the  fact  that  at  first, 
m  the  embryo  of  the  Amphioxus,  as  in  that  of  all  other 
Vertebrates,  the  side  wall  of  the  neck  is  perforated  in  such  a 
way  by  openings,  that  there  is  an  open  passage  through  the 
latter  from  the  external  skin  into  the  anterior  intestine 
(Fig.  158,  K).  The  inhaled  water,  which  is  taken  in  to  the 
gill-intestine  through  the  mouth,  passes  out  directly  through 
the  gill-openings.  While  the  number  of  these  gill-openings 
is  increasing  very  rapidly,  over  the  upper  row  of  these  a 
longitudinal  fold  rises,  on  each  side,  on  the  side-wall  of  the 
body  (Fig.  159,  £/").  The  narrow  body-cavity  prolongs  itself 
in  these  longitudinal  folds  (Lli).  Both  side-folds  grow 
downward  and  hang  as  free  gill-roofs.  The  free  edges  of 
these  then  incline  towards  each  other  and  coalesce  in  the 
middle  line  of  the  ventral  side,  thus  forming  the  ventral 
seam  or  Raphe  (Fig.  100,  K).  The  gill-pore  alone  remains 
open  (Fig  15,  p).  Thus  originates  a  closed  gill-cavity 
answering  exactly  to  that  of  Fishes,  and  at  the  same  time 
identical  with  that  of  the  Ascidians.  The  gill-cavity  of  the 
Ascidian,  the  Amphioxus,  the  Fishes,  and  the  larval  Am- 
phibia, are  to  be  regarded  as  homologous  parts.  This  large 
gill-cavity,  filled  with  water  and  communicating  freely 
with  the  surrounding  water,  must  be  distinguished  from 
the  small  body-cavity,  tilled  with  lymph  and  without  any 
external  communication.  The  latter,  the  cceloma  (Figs. 
158-160,  Mi),  in  the  adult  Amphioxus  is  very  narrow  and 
very  small  in  size  (Fig.  152,  LK).  When  the  gill-cavity 
of  the  Amphioxus  is  complete,  the  respiratory  water, 


452 


THE    EVOLUTION    OF    MAX. 


FIGS.  158-160. — Transverse  section  through  an  early  larval  form  of 
Amphioxus.  (Diagrammatic,  after  Eolph.)  (Cf.  Fig.  152,  p.  424.)  In  Fig. 
158  there  is  a  free  passage  from  without  into  the  intestinal  cavity  (D), 
through  the  gill-openings  (K).  In  Fig.  159  the  lateral  longitudinal  folds 
of  the  body-wall,  the  gill-roof,  are  forming,  growing  downwards.  In  Fig. 
160  these  side-folds  have  grown  towards  each  other  and  their  edges  have 


THE   LANCELET   AND   THE   PRIMITIVE   VERTEBRATE.     453 

woalesced  in  the  middle  line  of  the  ventral  side  (B).  The  respiratory  water 
now  passes  from'  the  intestinal  cavity  (.D)  into  the  gill-cavity  (W).  In  all, 
the  letters  indicate  the  same  parts  :  N,  medullary  tube ;  Ch,  notochord  ; 
M,  side-muscles ;  Lh,  body-cavity ;  O,  portion  of  the  body-cavity  in  which 
the  sexual  organs  afterwards  form  ;  D,  intestinal  cavity  lined  by  the  intes- 
tinal-glandular layer  (a);  A,  gill-cavity;  K,  gill-openings;  b  =  E,  outer  skin, 
or  epidermis  ;  £r,  the  same  as  the  inner  epithelium  of  the  gill-cavity;  F3, 
the  same  as  the  outer  epithelium  of  the  gill-cavity. 


which  was  taken  in  at  the  mouth,  passes  out,  no  longer 
directly  through  the  gill-openings,  but  through  the  gill-pore 
(p.  branchialis).  That  portion  of  the  intestinal  canal  which 
is  situated  behind  the  gill-body  becomes  the  stomach- 
intestine,  and  forms  on  the  right  side  a  single  purse-like 
protrusion,  which  becomes  a  blind  liver-sac.  This  digestive 
portion  of  the  intestinal  canal  is  enclosed  in  the  narrow 
body-cavity. 

In  an  early  stage  of  individual  development,  the  struc- 
ture of  the  body  of  the  Amphioxus  larva  still  corresponds 
essentially  with  our  ideal  "Primitive  Vertebrate."  The 
body  afterwards,  however,  undergoes  various  modifications, 
especially  in  the  anterior  portion.  These  modifications  are 
uninteresting  to  us  at  present,  because  they  depend  on 
special  conditions  of  Adaptation,  nor  have  they  anything  to 
do  with  the  hereditary  vertebrate  type.  Of  the  remaining 
portions  of  the  body  of  the  Amphioxus,  we  need  only 
remark  that  the  germ -glands,  or  internal  sexual  organs,  do 
not  deveope  till  later,  and,  as  it  appears,  directly  from  the 
inner  cell-coat  of  the  body-cavity,  from  the  ccelom- 
epithelium.  Although  no  extension  of  the  body-cavity 
is  afterwards  discernible  in  the  side  walls  of  the  gill-cavity, 
in  the  gill-roofs  (Fig.  152),  yet  such  an  extension  does  at 
first  exist  (Fig.  159,  160,  Lh}.  In  the  lowest  part  of  this 


454  THE   EVOLUTION   OF   MAN. 

extension,  the  sexual  glands  originate  from  a  portion  of 
the  ccelom-epithelium  (Fig.  160,  ff).  Tn  other  respects,  the 
farther  modification  of  the  larva  into  the  adult  form  of  the 
Amphioxus  is  so  simple  that  we  need  not  now  follow  it.121 

We  will  now  turn  to  the  history  of  the  development  of 
the  Ascidian,  an  animal  apparently  so  much  lower  and  so 
far  simpler  in  its  organization,  which  spends  the  greater 
part  of  its  life  as  an  unshapely  mass,  adhering  to  the  bottom 
of  the  sea.  It  was  most  fortunate  that  Kowalevsky  in  his 
researches  first  fell  in  with  those  larger  Ascidian  forms 
which  most  clearly  testify  to  the  kinship  between  Verte- 
brates and  Invertebrates,  and  of  which  the  larvae,  in  the 
first  stages  of  development,  are  exactly  similar  to  those  of 
the  Amphioxus.  This  agreement  in  all  the  essential  charac- 
ters is  so  great  that  it  is  really  only  necessary  to  repeat 
word  for  word  what  has  already  been  said  about  the 
Ontogeny  of  the  Amphioxus. 

The  egg  of  the  larger  Ascidia  (Phallusia,  Cynthia,  etc.) 
is  a  simple  globular  cell  ^  to  |-  mm.  in  diameter.  In  the 
cloudy,  finely  granular  yelk  a  bright,  globular  germ-vesicle 
(nucleus)  about  -£$  mm.  in  diameter  is  seen,  enclosing  a 
germ-spot  (nudeolus).  (Fig.  1,  Plate  X.)  Within  the  enve- 
lope, which  surrounds  the  egg,  the  parent-cell  of  the 
Ascidian,  after  fertilization,  passes  through  exactly  the 
same  changes  as  the  cytula  of  the  Amphioxus.  The  special 
incidents  in  the  fertilization  and  egg-cleavage  of  the  largest 
and  most  interesting  of  our  Ascidians  (Phallusia  mam- 
milata)  have  lately  been  very  accurately  studied  and 
described  by  Edward  Sbrasburger.  The  remarkable  details 
of  these  processes,  which  do  not,  however,  touch  our  present 
purpose,  are  given  in  the  excellent  work  by  that  writer 


GERM-HISTORY   OF   THE   ASCIDIAN.  455 

on  "Zellbildung."122  Here,  .as  in  the  Amphioxus,  the  germ- 
vesicle  (nucleus)  of  the  egg-cell  disappears  in  great  measure 
even  before  fertilization,  while,  after  the  latter  process  is 
accompUshed,  the  monerula,  in  consequence  of  the  re-forma- 
tion of  a  kernel,  becomes  a  cytula.  This  breaks  up  by 
primordial  cleavage  into  2,  4,  8,  16,  32  cells,  and  so  on.  By 
continued  total  cleavage  the  morula  forms  the  mulberry -like 
heap  of  like  cells.  Within  this  a  liquid  accumulates,  and 
thus  a  globular  germ-membrane  vesicle  is  once  more  formed, 
the  wall  of  which  consists  of  a  single  cell-stratum,  the 
blastoderm  (Plate  X.  Fig.  3),  just  as  in  the  case  of  the 
Amphioxus  a  true  Gastrula,  a  simple  Bell-gastrula  (Plate  X. 
Fig.  4),  is  formed  from  this  blastula  by  inversion. 

Up  to  this  point  in  the  evolution  of  the  Ascidian  there 
is  no  definite  ground  for  assuming  its  near  relationship  to 
the  Vertebrates ;  for  a  similar  Gastrula  arises  in  the  same 
way  in  the  most  diverse  animals  of  other  tribes  also.  Now, 
however,  comes  an  evolutionary  process  which  is  peculiar  to 
Vertebrates,  and  which  absolutely  demonstrates  the  kinship 
of  the  Ascidia  and  the  Vertebrates.  From  the  outer  skin 
(epidermis}  of  the  Gastrula  originates  a  medullary  tube, 
and,  between  this  and  the  primitive  intestine,  a  notochord 
— organs  which  otherwise  occur  only  in  Vertebrates,  and 
are  peculiar  to  them.  The  formation  of  this  highly  im- 
poi  tant  organ  takes  place  in  the  Gastrula  of  the  Ascidian 
exactly  as  in  that  of  the  Amphioxus.  In  the  Ascidian  also, 
the  oblong-round  or  oval  Gastrula-body,  which  has  but  a 
single  axis,  becomes  flat  on  one  side,  on  the  future  dorsal 
side.  Along  the  central  line  of  this  flat  side,  a  furrow  or 
trench  forms,  the  medullary  furrow,  and  on  either  side  of 
this  two  parallel  ridges  or  swellings  arise  from  the  skin- 


456  THE   EVOLUTION    OF   MAN. 

layer.  These  two  medullary  swellings  coalesce  over  the 
furrow,  thus  forming  a  tube ;  in  this  case  also,  this  nerve 
tube  or  medullary  tube  is  originally  open  in  front,  but 
closed  behind.  Again,  in  the  Ascidian  larva  also,  the  per- 
manent mouth-opening  is  a  new  formation,  and  does  not 
originate  from  the  primitive  mouth  .of  the  Gastrula;  the 
latter  closes,  and  in  its  neighbourhood  the  future  anal 
opening  is  formed  by  inversion  from  the  outside,  at  the 
opposite  end  from  the  opening  of  the  medullary  tube  (Plate 
X.  Fig.  5,  a). 

While  these  important  changes  are  taking  place,  exactly 
in  the  same  way  as  in  the  Amphioxus,  a  tail-like  appendage 
grows  out  from  the  posterior  end  of  the  larval  body,  and 
the  larva  curls  itself  within  the  spherical  egg-covering  in 
such  a  way  that  its  dorsal  side  projects,  while  the  tail  is 
bent  back  upon  the  ventral  side.  In  this  tail  now  de- 
velops a  cylindrical  cord,  composed  of  cells,  the  anterior 
end  of  which  extends  into  the  body  of  the  larva  between 
the  intestinal  and  the  medullary  tubes :  this  is  the  chorda 
dorsalis,  an  organ  which,  except  in  this  one  case,  is  found 
only  in  Vertebrates,  and  of  which  no  other  trace  is  to  be 
seen  in  Invertebrates.  Here,  again,  the  notochord  consists, 
at  first,  of  a  single  row  of  large  bright  cells  (Plate  X.  Fig. 
5,  c/t);  afterwards  it  consists  of  several  cell-rows.  So,  too,  in 
the  Ascidian  larva,  the  notochord  develops  from  the  middle 
portion  of  a  cell-stratum,  the  side  portions  of  which  become 
tail-muscles,  and  which  can,  therefore,  only  be  the  skin- 
librous  layer.  At  the  same  time,  a  cell-stratum  splits  oti 
from  the  intestinal  wall,  which  afterwards  forms  the  heart, 
the  blood  and  the  vascular  system,  and  also  the  intestinal 
muscles.  This  is  the  intestinal-fibrous  layer. 


FREE   ASCIDIAN  LABV.E. 


457 


On  making  a  section  through  the  middle  of  the  body  in 
this  stage  (at  the  point  where  the  tail  joins  the  trunk),  we 
find  in  the  Ascidian  larva  precisely  the  same  characteristic 
disposition  of  the  chief  organs  as  in  the  larva  of  the 
Amphioxus  (Plate  X.  Fig.  6).  In  the  middle,  between  the 
medullary  tube  and  the  intestinal  tube,  is  the  chorda  dor- 
salis ;  and  on  each  side  of  the  latter,  the  muscle-plates  of 
the  back.  The  section  of  the  Ascidian  larva  now  differs  in 
no  essential  way  from  that  of  our  ideal  Vertebrate  (Fig. 
161). 

When  it  has  reached  this  stage  of  development,  the 
Ascidian  larva  begins  to  move  within  the  egg-covering. 
This  ruptures  the  egg-covering ;  the  larva  emerges  from  the 
latter,  and  swims  freely  about  in  the  sea  by  means  of  its 
rudder-like  tail  (Plate  X.  Fig.  5).  These  free-swimming 
Ascidian  larva  have  long  been  known  to  science.  They 
were  first  observed  by  Darwin  during  his  voyage  round  the 
world  in  1833.  In  external  form  they  resemble  the  larva 
of  the  frog,  the  tadpole,  and  they  move  about  in  the  water 


FIG.  161. — Transverse  section  through  ideal 
Primitive  Vertebrate  (Fig.  52).  The  section 
passes  through  the  sagittal  axis  and  the  cross 
axis  :  n,  medullary  tube ;  x,  notochord ;  t,  dorsal 
vessel ;  v,  ventral  vessel ;  a,  intestine ;  c,  body, 
cavity;  ml,  dorsal  muscles;  m2,  ventral  mus- 
cles ;  h,  outer  skin. 


like  the  latter,  using  their  tail  as  a  rudder.  This  highly 
developed  youthful  condition  of  free  movement  lasts,  how- 
ever, only  for  a  short  time.  A  further  progressive  develop- 


458  THE   EVOLUTION    OF   MAN. 

ment  yet  occurs ;  two  small  sense-organs  make  their  appear- 
ance in  the  foremost  part  of  the  medullary  tube :  of  these 
the  one  is,  according  to  Kowalevsky,  an  eye,  the  other  an 
organ  of  hearing  of  the  simplest  structure.  A  heart  also 
develops  on  the  ventral  side-  of  the  animal,  on  the  lower 
wall  of  the  intestine ;  and  this  is  of  the  same  simple  form, 
and  is  situated  in  the  same  place  as  the  heart  in  Man  and 
all  other  Vertebrates.  In  the  lower  muscle-wall  of  the 
intestine  a  wart-like  growth  makes  its  appearance — a  solid 
spindle-shaped  cord  of  cell, — the  interior  of  which  soon 
becomes  hollow :  it  begins  to  move  by  contracting  in  oppo- 
site directions,  now  backwards,  and  then  again  forwards,  as 
in  the  full-grown  Ascidian.  In  this  way  the  blood-fluid, 
collected  in  the  hollow  muscular  pouch,  is  driven  in  alter- 
nate directions  into  the  blood-vessels,  which  develop  at  both 
ends  of  this  tubular  heart.  A  main  vessel  traverses  the 
dorsal  side  of  the  intestine,  another  its  ventral  side ;  the 
former  represents  the  aorta  (Fig.  161,  t}  and  the  dorsal  vessel 
of  Worms.  The  latter  represents  the  intestinal  vein  (Fig. 
161,  v)  and  ventral  vessel  of  Worms. 

When  these  organs  are  complete,  the  progressive  Onto- 
geny of  the  Ascidian  is  at  an  end,  and  retrogression  now 
commences.  The  freely-swimming  Ascidian  larva  sinks  to 
the  bottom  of  the  sea,  relinquishes  its  power  of  free  loco- 
motion, and  becomes  fixed.  By  means  of  that  very  part 
of  its  body  which  was  foremost  in  locomotion,  it  adheres 
to  stones,  marine  plants,  shells,  corals,  and  other  objects  at 
the  bottom  of  the  sea.  To  secure  it  to  these,  several 
excrescences  are  employed,  usually  three  wart-like  bodies, 
which  may  be  observed  on  the  larva,  even  while  it  yet 
swims.  The  tail,  which  is  of  no  further  use,  is  now  lost 


APPENDICULARIA. 


459 


It  undergoes  fatty  degeneration,  and  is  cast  off  together 
with  the  entire  notochord.  The  tail-less  body  becomes  a 
shapeless  bag,  or  sac,  which,  by  retrograde  metamorphosis 
of  its  separate  parts  and  by  re-formation  and  modification, 
gradually  acquires  that  remarkable  structure  which  has 
already  been  described 

FIG.  162.— Appendicularia  (Copelata), 
seen  from  the  left  side  :  mi,  mouth;  k,  gill- 
intestine  ;  o,  oesophagus ;  v,  stomach ;  a, 
anus ;  n,  brain  (upper  throat  ganglion) ; 
g,  ear-vesicle ;  /,  groove  under  the  gill ; 
h,  heart ;  t,  testes  ;  e,  ovary ;  c,  notochord ; 
*,  tail. 

Among  the  extant  Mantle 
Animals  (Tunicata)  -there  is,  how- 
ever, an  interesting  group  of 
small  animals  which  retain 
throughout  life  the  tailed,  inde- 
pendent ascidian  larval  stage  of 
development,  and  which,  by 
means  of  their  permanent,  broad, 
rudder-like  tails,  move  actively 
about  in  the  sea.  These  are  the 
remarkable  Appendi  cularice  (Fig. 
1(52).  They  are  the  only  extant 
Invertebrates  permanently  pos- 
sessing a  notochord,  and  are, 
therefore,  the  nearest  allies  of 
the  extinct  Chorda  Animals 
(Ckordonia),  of  the  primaeval 

Worms  which  must  be  regarded  as  the  common  parent-form 
32 


460  THE   EVOLUTION   OF   MAN. 

of  Mantle  Animals  (Tunicata)  and  of  Vertebrates.  The 
notochord  of  the  Appendicularia  is  a  long  cylindrical  cord 
(Fig.  162,  c),  which  serves  to  connect  the  muscles  which 
move  the  flat,  rudder-like  tail. 

Among  the  various  retrogressions  which  are  undergone 
by  the  Ascidian  larva  after  it  has  attached  itself,  the 
degeneration  of  one  of  the  most  important  parts  of  the 
body,  the  medullary  tube,  is,  next  to  the  loss  of  the  noto- 
chord, of  peculiar  interest.  While  in  the  Amphioxus  the 
medulla  steadily  develops,  that  of  the  Ascidian  larva  soon 
shrinks  to  the  proportions  of  a  small,  insignificant  nerve 
ganglion,  which  lies  over  the  mouth-opening,  above  the 
gill-body,  and  which  represents  the  exceedingly  low  mental 
endowments  of  this  animal  (Plate  XI.  Fig.  14,  m).  This 
insignificant  remnant  of  the  medullary  tube  seems  to  retain 
no  likeness  to  the  medulla  of  Vertebrates,  although  it 
originated  from  the  same  rudiment  as  the  medulla  of  the 
Amphioxus.  The  sense-organs,  which  had  developed  in  the 
anterior  end  of  the  nerve-tube,  are  also  lost ;  in  the  full- 
grown  Ascidian  there  is  no  trace  of  them.  On  the  other 
hand,  the  intestinal  canal  now  develops  into  a  very 
capacious  organ.  This  soon  breaks  up  into  two  separate 
parts — a  wide  anterior  gill-intestine  for  respiration,  and  a 
narrow  posterior  stomach-intestine  for  digestion.  In  the 
former  the  gill-openings  form  in  exactly  the  same  way  as 
in  the  Amphioxus.  At  first  the  number  of  gill-openings  is 
very  small ;  it  afterwards,  however,  increases  considerably, 
and  gives  rise  to  a  large,  lattice-like  perforated  gill-body. 
The  "  hypobranchial  groove "  originates  in  the  central  line 
of  the  ventral  side  of  this  gill-body.  The  wide  gill-cavity, 
which  surrounds  the  gill -body,  also  develops  in  the  Ascidian 


RETROGRESSIVE   DEVELOPMENT.  461 

f 

just  as  in  the  Amphioxus.  The  excretory  opening  of  the 
former  conesponds  fully  to  the  abdominal  pore  of  the  latter. 
In  the  adult  Ascidian  the  gill-intestine  and  the  heart  rest- 
ing on  the  ventral  side  of  the  latter,  are  almost  the  only 
organs  that  recall  the  original  relationship  to  Vertebrates. 

In  conclusion  we  will  glance  at  the  development  of  the 
curious  external  gelatinous  mantle,  or  cellulose  sac,  in  which 
the  Ascidian  is  afterwards  entirely  enclosed,  and  which 
characterizes  the  whole  class  of  Mantle  Animals  (Tunicata). 
Very  various  and  remarkable  views  have  been  entertained 
as  to  the  formation  of  this  mantle.  For  instance,  it  was  the 
opinion  of  Kowalevsky,  that  the  animal  does  not  itself 
form  the  mantle,  but  that  the  latter  is  produced  by  special 
cells  from  the  maternal  body,  which  surround  the  egg. 
According  to  this  the  mantle  would  be  a  permanent 
egg-envelope.  This  is  contrary  to  all  analogy,  and  d 
priori  highly  improbable.  Another  naturalist,  Kupffer, 
who  has  confirmed  and  extended  the  researches  of  the 
former,  assumed  that  the  mantle  develops  from  cells  which, 
even  before  the  impregnation  of  the  egg-cell,  form  from  the 
outer  portion  of  the  yelk,  and  separate  entirely  from  the 
inner  portion.  This  seems  very  doubtful  and  unlikely. 
Hertwig's  researches,  which  are  confirmed  by  my  own 
observations,  first  showed  that  the  mantle  develops  as  a 
so-called  "cuticula."  It  is  an  exudation  from  epidermic 
cells,  which  soon  hardens,  separates  from  the  real  body  of 
the  Ascidian,  and  condenses  so  as  to  form  a  strong  envelope 
round  the  latter.  The  matter  of  these  cells  is  chemically 
indistinguishable  from  the  cellulose  of  plants.  While  the 
epidermic  cells  of  the  external  horn-plate  are  secreting  this 
mass  of  cellulose,  some  of  them  drop  into  it,  continue  to 


462  THE   EVOLUTION    OF   MAN. 

live  in  the  exuded  mass,  and  aid  in  constructing  the  mantle. 
In  this  way  the  strong  external  covering  is  at  length 
formed,  grows  thicker  and  thicker,  and  in  many  adult 
Ascidia  constitutes  upwards  of  two-thirds  of  the  entire  mass 
of  the  body.123 

The  farther  development  of  the  individual  Ascidian  is 
of  no  special  interest  to  us,  and  we  will  therefore  not  continue 
to  trace  it.  The  most  important  result,  supplied  by  Onto- 
genesis, is  its  perfect  agreement  with  that  of  the  Amphioxus 
in  the  earliest  and  most  important  stages  of  its  germ- 
history.  It  is  only  after  the  medullary  and  intestinal  tubes, 
and,  between  these,  the  notochord  with  its  muscles,  have 
been  formed,  that  their  development  takes  different  direc- 
tions. The  Amphioxus  pursues  a  steadily  progressive  course 
of  development,  till  it  entirely  resembles  the  parent-forms 
of  the  higher  Vertebrates,  while  the  Ascidian,  on  the  con- 
trary, enters  on  a  course  of  retrograde  metamorphosis,  and 
finally,  in  the  developed  state,  appears  as  a  very  imperfect 
member  of  the  Worm  group. 

Those  who  again  review  all  the  remarkable  facts  which 
we  have  found  both  in  the  structure  and  in  the  germ- 
history  of  the  Amphioxus  and  Ascidian,  and  who  then 
compare  these  with  the  previously  ascertained  facts  of 
human  germ-history,  will  not  think  that  I  have  ascribed 
exaggerated  importance  to  these  highly  interesting  animal 
forms.  For  it  is  now  evident  that  the  Amphioxus  as  the 
representative  of  Vertebrates,  and  the  Ascidian  as  the  repre- 
sentative of  Invertebrates,  form  the  bridge  which  alone  can 
span  the  deep  gulf  between  these  two  main  divisions  of  the 
animal  kingdom.  The  fundamental  agreement  exhibited 
by  the  Lancelot  and  the  Ascidian  in  the  first  and  the  most 


THE  LANCELET  AS  THE  MOST  ANCIENT  VERTEBRATE.  463 

important  points  of  their  embryonic  development  does  not 
only  testify  their  close  anatomical  form-relationship  and 
their  connection  in  the  system;  it  also  testifies  their  true 
blood-relationship  and  their  common  origin  from  one  and 
the  same  parent  form;  and  hence  it  at  the  same  time 
throws  a  flood  of  light  upon  the  earliest  origin  of  human 
genealogy.124 

Writing  in  1868  "  on  the  origin  and  genealogy  of  the 
human  race,"  I  insisted  upon  the  extraordinary  importance 
of  this  circumstance,  and  declared  that  we  must  accordingly 
"regard  the  Amphioxus  with  special  veneration  as  that 
animal  which  alone  of  all  extant  animals  can  enable  us  to 
form  an  approximate  conception  of  our  earliest  Silurian 
vertebrate  ancestors."  This  proposition  has  given  very 
great  offence,  not  only  to  unscientific  theologians,  but  also 
to  many  others,  especially  such  philosophers  as  still  cherish 
the  anthropocentric  error,  and  who  look  on  man  as  the  fore- 
ordained object  of  "  creation,"  and  as  the  true  final  cause  of 
all  terrestrial  life.  The  "  dignity  of  humanity,"  it  was  said 
in  a  church  newspaper,  is,  by  such  a  statement  as  mine, 
"trodden  underfoot,  and  the  divine  rational  conscience  of 
man  grievously  hurt." 

This  indignation  at  my  honest  and  deep  respect  for  the 
Amphioxus  is,  I  am  free  to  confess,  quite  incomprehensible 
to  me.  If,  on  entering  a  grove  of  ancient  oaks,  we  express 
reverence  for  these  venerable  trees,  the  life  of  which  has 
endured  a  thousand  years,  no  one  thinks  this  unnatural 
Yet  how  high  above  the  oak  does  the  Amphioxus,  or  even 
th?  Ascidian  organization,  stand  in  this  respect !  And  what 
are  the  thousand  years  of  life  of  a  venerable  oak  compared 
with  the  many  millions  of  years  the  history  of  which  is  told 


464  THE   EVOLUTION    OF   MAN. 

by  the  Amphioxus !  But  apart  from  all  this,  the  Amphi- 
oxus  (skull- less,  brainless,  and  memberless  as  it  is)  deserves 
all  respect  as  being  of  our  own  flesh  and  blood !  At  any 
rate,  the  Amphioxus  has  better  right  to  be  an  object  of 
profoundest  admiration  and  of  devoutest  reverence,  than  any 
one  in  that  worthless  rabble  of  so-called  "  saints  "  in  whose 
honour  our  "civilized  and  enlightened"  cultured  nations 
erect  temples  and  decree  processions. 

The  infinite  importance  of  the  Amphioxus  and  the  Ascidian 
as  explaining  the  development  of  Man,  and  consequently  his 
true  nature,  may  be  clearly  seen  from  the  following  sum- 
maries, in  which  I  have  stated  the  principal  homologies  of 
the  highest  and  of  the  lowest  Vertebrates  (Table  IX.).  The 
table  exhibits  the  undeniable  fact  that  the  human  embryo 
at  an  early  period  of  its  development  agrees  in  the  most 
essential  points  of  its  organization  with  the  Amphioxus  and 
with  the  embryo  of  the  Ascidian,  while,  on  the  other  hand, 
it  differs  radically  from  the  developed  Man.  It  is,  however, 
equally  important  that  we  should  remember  the  profound 
gulf  which  separates  the  Amphioxus  from  all  other  Verte- 
brates. Even  yet  the  Lancelet  is  represented  in  all  text- 
books of  Zoology  as  a  member  of  the  Fish  class.  .When  (in 
1866)  I  totally  separated  the  Amphioxus  from  the  Fishes,  and 
divided  the  entire  vertebrate  tribe  into  two  chief  groups,  the 
Skull-less  Animals  (Amphioxus)  and  the  Skulled  Animals 
(all  other  Vertebrates),  my  classification  was  regarded  as  a 
useless  and  unfounded  innovation.115  How  the  matter 
stands  is  best  seen  in  the  appended  table  (Table  X).  In  all 
essential  points,  Fishes  are  more  nearly  allied  to  Man  than 
to  the  Amphioxua. 


(    465     ) 


TABLE    IX. 

Systematic  Survey  of  the  most  important  homologies  between  the  human 
embryo,  the  embryo  of  the  Ascidian,  and  developed  Amphioxus  on  the 
one  hand,  and  on  the  other  hand,  the  developed  Man. 


Embryo  of 

Aacidian. 

Developed 
Amphioxua. 

Human 
Embryo. 

Developed 
Man. 

I. — Products  of  tho  Differentiation  of  the  Skin-layer. 


Naked  outer  skin 

Naked  outer  skin 

Naked  outer  skin 

Hairy  outer  skin 

Simple  medullary 

Simple  medullary 

Simple  medullary 

Brain    and  spinal 

tube 

tube 

tube 

marrow 

Primitive  kidney  (?) 

Primitive  kidney  (?) 

Primitive    kidney 

Oviduct  and 

(excretory  canal?) 

duct 

sperm-duct 

Simple  thin  leather 
skin  (Corium) 

Simple  thin  leather 
skin  (Corium) 

Simple  thin  leather 
skin  (Corium) 

Differentiated 
thickleatherskin 

(Cortum) 

Simple  skin-mus 

Simple  trunk- 

Simple  muscle- 

Differentiated 

cular  pouch 

muscle  system 

plate 

trunk-muscle 

system 

Notochord 

Notochord 

Notochord 

Vertebral    column 

No  skull 

No  skull 

No  skull 

Bony  skull 

No  limbs 

No  limbs 

No  limbs 

Two  pair  of  limbs 

Hermaphrodite 

Separated  sexual 

Hermaphrodite 

Separated    sexual 

sexual  glands 

glands 

sexual  glands 

glands 

IL — Products  of  the  Differentiation  of  the  Intestinal  layers. 


Simple  body  cavity 
(Cceloma) 

Simple  body  cavity 
(Cceloma) 

Simple  body  cavity 
(Cceloma) 

Distinct  chest  and 
ventral  cavitiea 

One-chambered 

Simple  tubular 

One-chambered 

Four-chambered 

heart 

heart 

heart 

heart 

Dorsal  vessel 

Aorta 

Aorta 

Aorta 

Simple  liver  pouch 

Simple  liver  pouch 

Simple  liver  pouch 

Large  differen- 

(?) 

tiated  liver 

Simple    intestinal 

Simple    intestinal 

Simple    intestinal 

Differentiated  in- 

tube    with  gill- 

tube    with  gill- 

tube  with   gill- 

testinal       tube 

openinge 

openinge 

openinga 

without       gill. 

openings 

466 


THE    EVOLUTION    OF   MAN. 


TABLE   X. 

Systematic  Survey  of  the  points  of  connection  in  form  of  the  Ascidiau  aud 
Amphioxus  on  the  one  side,  and  the  Fishes  and  Men  on  the  other,  in 
completely  developed  conditions. 


Developed 
Ascidiam,. 

Developed 
Amphioxus. 

Developed 
Fish. 

Developed 
Man. 

Head    and    trunk 

Head    and    trunk 

Head    and    trunk 

Head    and    trunk 

not  distinct 

not  distinct 

distinct 

distinct 

No  limbs 

No  limbs 

Two  pair  of  limbs 

Two  pair  of  limba 

No  skull 

No  skull 

Developed  skull 

Developed  skull 

No  tongue-bone 

No  tongue-bone 

Tongue-bone 

Tongue-bone 

No  jaw-apparatus 

No  jaw-apparatus 

Jaw-apparatus 

Jaw-apparatus 

(upper        and 

(upper        and 

lower  jaws) 

lower  jaws) 

No   vertebral 

No   vertebral 

Articulated  verte- 

Articulated verte. 

column 

column 

bral  column 

bral  column 

No  ribs 

No  ribs 

Ribs 

Ribs 

Brain    undifferen- 

Brain    nndifferen- 

Brain    differen- 

Brain   differen- 

tiated _ 

tiated 

tiated 

tiated 

Eyes  rudimentary 

Eyes  rudimentary 

Eyes  developed 

Eyes    developed 

No  ear-organ 

No  ear-organ 

Ear  -  organ     with 

Ear-organ       with 

three     semicir- 

three    semicir- 

cular canals 

cular  canals 

No      sympathetic 

No      sympathetic 

Sympathetic 

Sympathetic 

nerve 

nerve 

nerve 

nerve 

Intestinal    epithe- 

Intestinal  epithe- 

Intestinal  epithe- 

Intestinal epithe- 

lium ciliated 

lium  ciliated 

lium  not  ciliated 

lium  not  ciliated 

Simple    liver    (or 

Simple  liver  (blind 

Compound      liver 

Compound      liver 

none) 

intestine) 

gland 

gland 

No    ventral    sali- 

No   ventral    sali- 

Ventral    salivary 

Ventral     salivary 

vary  gland 

vary  gland 

gland 

gland 

No  swimming 

No    swimming 

Swimming     blad- 

Lungs (swimming 

bladder 

bladder 

der     (rudimen- 

bladder) 

tary  lungs) 

Kidneys  rudimen- 

Kidneys rudimen- 

Kidneys deve- 

Kidneys  deve- 

tary (?) 

tary  (?) 

loped 

loped 

Simple   heart 

Simple       tubular 

Heart  with  valves 

Heart  with  valves 

pouch 

heart 

and  chambers 

and  chambers 

Blood  colourless 

Blood  colourless 

Blood  red 

Blood  red 

No  spleen 

No  spleen 

Spleec 

Spleen 

Hypobranchial 

Hypobranchial 

Thyroid  gland 

Thyroid  gland 

groove  on  gill- 

groove  on   gill- 

body 

body 

(    467    ) 


TABLE    XI. 

Systematic  Survey  showing  the  derivation  of  the  germ-layers  of  the  Amphioxur 

from  the  parent-cell  (cytula),  and  of  the  main  organs  from  the  germ-layeig. 

(Tree  showing  the  ontogenetic  descent  of  the  cells  in  the  Amphioxus).125 


Medullary  tube  ' 

Tubus  medallarit     Flech 

mass 

I 

Hood-vessels 

Mas 

cult 

Canales        G 

ill-epithelium 

Notochord 

tai.gue/eri    Epith.'Uranckiate 

Chorda, 

Stomach 

8">nee-  organs 

epithelium 

Scnsuaria 

%^ 

v  

-"* 

Int 

stinal 

Liver 

Small 

m 

uscle 

epi- 

intestine 

Leather  si 

da 

Testes    Ovaries 

v 

•all 

thelium 

epi- 

Outprskfn 

Cerium 

• 

Testiculi  Ovaria 

F 

eri- 

thelium 

fyidermia 

ent 

erum 

1  

1 

Primitive 

Parietal 
Coelom- 

Viscpral 
Coelom- 

Mesen- 
tery 

Epithelium 

kitli  eys 

E! 

>i- 

EI 

>i- 

Mesen- 

Pr<>- 

the 

nm 

the! 

um 

terium 

tonephra 

OM 

<:IK- 

End 

>   (C- 

1 

lar 

um 

tar 

1  — 

_* 

Y  

^S 

Skin-sensory  Skin-fibrous 

layer  layer 

imina  neurodermalis)  (/ximina  inofei malts') 

(Fig.  157,  tin)  (Fig.  157,  hf) 


a  (Fig.  155,  e) 
(Skiu-layer,  or  outer  germ-layer; 


Intestinal-fibrous   Intestinal-g]an- 

layer  dular  layer 

(Lamina  inogastralis)  (lM.minamyxogaslralii\ 
(Fig.  157,  dj  )  (Fig.  157,  dd) 


"Fnto-erma  (Fig-  iss,  t) 
(Intestinal  liyer,  or  inner  germ-layer; 


Giatrula  TFi-  22  /,  K    ' 
(Cup-germ)  [_Fig.  155>  P-  444  J 


stul 


(Bladder-germ) 


ig.  22,  F,  G) 


Mo-ula  (Fig.  22,  J?) 
(MuD)erry-germ) 

Cyfu1a  (Fig.  22,  fl) 
(Parent-cell) 


Mo^erula  (Fig.  iffl,  4) 
(P/ireE 


Bnt-cytud') 


v,  I 


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